I'm sure now because he's doing what I think is mating behavior. Though I don't know if I should be worried because I still don't know Pip and Kiwi's sex. This is what bothers me: he'll try to mate with one of them if one of them is a female and I still don't want that, just like eddie...

Here's a video, and PS: sorry for the stained window, i was washing the dishes and made a mess:

Before I address your concern, there's something I myself is concerned about.

It's the cage location. I do remember you saying that the cage goes outside, but is there at least a roof over their heads? Even with clipped wings, cockatiels can fly surprisingly well if their wings are strong enough. In fact, Lucy's wings were clipped when she flew away. Since Coco also came from Carolyn, I would expect him to be a strong flyer, as well.

Anyways, in my experience, I found that male cockatiels will sing and heart wing (which is what they do when they open their wings like that) to just about anything. This will include birds of the same gender. I had two males, and one would sing to his brother all the time. And then I had a male that would sing and heartwing to nothing in particular. It's not really a behavior that's exclusive to mating. It's just something male cockatiels do basically to show off, even when there is no mate available.

But if you are concerned about breeding, then you could just try and keep Coco separated from them until you figure out who's the female. I remember you saying before that he's usually out all day and has a small cage for sleeping in, correct? So you might already be ok.

In the video, Kiwi doesn't look interested. I would only be concerned if either pip or kiwi showed body language that meant they are ok with Coco mounting them.
I'll use eddieparrot5's video as an example (hope you don't mind, Eddie).

Hey look, the [video][/video] thing works for photobucket too!

Anyways, while I don't usually see them walk side to side like that, female cockatiels will usually hold that pose when they want to mate.

Oh, I thought that he wanted to mate. All my birds are outside, and not because I want to, but because I have no permission to keep their cage inside. Though their wings are very trimmed, and I wouldn't expect them to fly away with those tiny things. But I understand things CAN happen, so I'm always around whenever they're outside their cage. Also, there is a roof over them, but there is no roof next to the metal door shown in the video.

That's male behavior all right!

Another risk to having them out of the cage outdoors like that is that a predator (like a dog, cat or wild animal) could come in and take them pretty easily. But you know more about the safety conditions near your house than we do.

I know that it is very unsafe for them to be outside. But unfortunately, I can't do much about it. My parents gave me strict permission on having them on that exact location. They are very stubborn when it comes to change because they are rooted to their traditions and what everybody thinks, so it is nearly impossible to even talk to them. They say I'm lucky I even have them, wich is kinda true. But I don't know what I could do to improve my birds wellbeing if I am very restricted in terms of what I can do with my birds. This is why I was considering giving 2 of them away, I can't give them what I want, I can't give them, the best.

They are very pretty!

One thing you could do in place of out-of-cage time is get a big flight cage, so they have plenty of room to stretch their wings and exercise, but not have to come out. They're pretty pricey, but if your parents are willing to let you get a cage of such a size, then it's definitely worth it for the birds.

You could probably build something big enough for them as well, for a cheaper price. Hardest part would probably either be making a design plan, or finding the right materials (or building it, depending on how good you are at building stuff).

Its good that you want what is best for them which giving them away would be a good thing for them if they are giving to the right person of course.To me keeping them in conditions like that because they aren't allowed inside is kind of selfish.But if it were me in your situation I would have sneaked them to my room lol or I would tell my parents that I would clean up after them and try keep the noise down.