Breeding siblings is generally considered to be the worst possible way to pair closely related birds. Even show breeders who do line breeding discourage it, and say that it's better to pair a parent and child than to pair two siblings.

It might be that she's feeling too uncertain about her nest area to lay any more eggs right now. You blocked her off from the place that she likes, and she hasn't found a substitute yet. Taking eggs away sometimes does make them lay more, but having a secure area where they can lay eggs is an important factor.

Yesterday I blocked that place before leaving to work but she managed to get in while I was at work. She sat on eggs all day yesterday. At night, I managed to put Bella into the cage (they all sleep together in a large cage) so the eggs were abandoned overnight. She was somewhat aggressive when I did this, but not super aggressive.
Today again I left to work and blocked that place, being totally unsure if this is what I need to do. I guess I don't dead-block it with a brick. Just came home for lunch and she is in the shoe cubby again. I put a large bag to block the entrance and a thick magazine to hold it in place
Dummyeggs website has lots of info about egg laying and they suggest not to deprive your birds' maternal instincts (they say let them sit on dummy eggs). On my question: won't they become depressed and pluck if their dummy eggs never hatch, their answer was "Not really. They are programmed to just keep procreating. They don’t really count the living relatives" which can be very convenient from the egg-selling prospective.
I can't believe that I did such stupid thing as letting her to her eggs on Tuesday when I wasn't sure if she was about to lay another egg.
I guess for now, I should let her sit on her 2 eggs. I plan to boil her 2 eggs tonight. shouldn't be too late, plus she wasn't sitting on them for 2 last nights. The location in the shoe cubby is good from one hand, because the rest of the flock doens't usually hang out in that area at all. Im not sure what to do about night time... thinking about moving her nest into location where she can spend a night with her eggs (right?)
My goal would be to prevent future clutches / egg laying. Don't want a constant egg disaster even if I don't have to worry about hatchlings (if using dummy eggs or boiled eggs) .
Please let me know your opinion... I don't want to be mean to Bella.

They're right that cockatiels usually don't get depressed when their eggs don't hatch - they just lay some more. So you need to try to get her hormonal urges under control as soon as you can, because the eggs she is sitting on right now aren't going to hatch. The "due date" for eggs will vary somewhat depending on the temperature and other conditions, but with my birds the eggs hatch 16 to 18 days after incubation began. A cockatiel hen will often start to lay a new clutch about a week after the first one failed to hatch.

Your bird's incubation has been interrupted so I don't know exactly how she will "count" the days. Birds can't actually count anyway, and don't even know how many eggs they laid - they just know instinctively when it's been too long. You need to get her out of breeding mode before she starts laying a second clutch.

Carolyn, I appreciate your advice!
this morning I boiled her 2 eggs. One was floating in water, the other sank (what does it mean when they float?) eggs didn't seem to be damaged but lost their pinkish hue after being boiled.
Later today, we had a surprise when Bella laid another egg... she decided to add to the clutch after sitting on 2 eggs for 3 days.
She still lets me pick her up when its a bedtime... not happy but she lets me. When put into the cage with other birds, she is a bit aggressive to them but stops soon.

Supposedly the fresher eggs will sink and older eggs will float. I don't know how reliable this is, especially with fertilized eggs. It's probably related to the size of the air cell inside the egg. Eggs lose moisture over time due to evaporation through the shell, so an older egg will have a bigger air cell than a newer one.

This makes sense.
Have you ever dealt with dummyeggs people? They have instructions on their site and it says that its OK to give a full clutch of eggs when your bird is only starting to think about laying the first egg... Hmmm... would she believe its her eggs if she hasn't laid any ? (I have 5 females... would each of them think its her eggs LOL?)
it also says if one or two are laid, you should replace these real eggs with a full clutch of dummies. Opinions?
I received dummies. they are slightly larger than our eggs

PS I still take Bella away from her eggs to the cage for the night. Since we are not talking about eggs' survival (supposedly there is no survival), what is better for her? let her sit properly - day and night? I want her to be OK - physically and mentally. We have 3 eggs (her own) and sitting since Tuesday, days only

I have two sets of their cockatiel sized eggs. Whether the bird believes it or not depends on the bird. Shodu didn't believe it, and would lay exactly five eggs no matter how many or how few eggs were in the nest. I could take several of the real eggs away and she would still stop after laying five, even if there were only two in the nest. But some hens do believe it.

How do they usually incubate at night? do they take turns or both sit there?
I still take Bella away from eggs to sleep with the flock. Usually in the morning she starts screaming to let me know its time to get her out.
Im thinking how to organize that she incubates at night too (just for her sake). I really don't want to let her stay in the shoe cubby where her eggs are, which is by the front door, overnight. The front door could be insulated better, when its cold there could be a draft where she sits.
I don't have too many options.
1) to move her whole nest (probably put eggs into shallow cardboard box first) into the living room under the table and just leave it there full-time. Cons - it won't be any longer hidden from the rest of the flock. It will give a good location for future egg laying which im trying to avoid. And finally, the parents won't be contained in the cage overnight which can be dangerous in case they are spooked. Oh, and if its in the living room then it will limit my late evening/ early morning activities in the living room.
2) at nights only move eggs that are put into a shallow cardboard box into their play daytime cage. That location won't work during the day because the flock actively uses the cage to play, eat, etc. So in the morning the eggs would need to be moved back into the shoe cubby...and so on. This way the parent would be contained in the cage overnight. Cons - constant moving of eggs back and forth. If I go with this option, do I lock both of them overnight with eggs or just the mom?
The male (Sparkle) does a good fair share of sitting during the day.

The most common incubation pattern is for the male to sit on the eggs in the daytime and the female to sit at night. These eggs aren't going to hatch, so it doesn't matter whether they get incubated at night. Moving the nest might cause more problems than it solves - she might not consider the new location to be her nest, and might not recognize that these are her eggs.

P.S. it's good to send her the message that she doesn't really have a suitable nesting place. The outcome that we want is for her to give up on nesting when this clutch doesn't hatch. We don't want her to think "I don't know what went wrong, but this is obviously a great nesting place so I'll lay some more eggs and hope that they hatch."