I had the same problem with my cousin, luckily she is starting back to grow up. They all grow up, sometimes it take such a long time. For your brother do what the first person said "anonymous" if you have a brother, act like their not bothering you. Honestly, I usually can't do that, I end up bursting into tears or hitting them because my rage builds up, don't judge me I can't help my body xD. As for a sister, well people the same gender as you can be difficult. They will steal your stuff, as the youngest I know how to do that very well, it's my job to. They will as I said steal your stuff, wear your makeup, and in some cases be jealous of you and make negative comments to make them feel better about themselves, but that is usually a sign of self esteem issues. Most people say to talk about this to a parent, most parents say to just set a good example. I recommend if it is that serious, talking to a counselor of some sort. I am sorry this has been so long
(EDIT) Oh and if they try to annoy you while your on your phone, get an old book you won't read again and cut a hole in it big enough to fit your whole phone and not make it noticeable. If you can or want to, tell me what your brother or sister or both does.
Ok. If you're 12 years old, you're too young for this site. You need to be 13 years old. It's for your protection.
If you're not trolling the site (kids tend to do that), then I would suggest that you play with her every day; read to her; play a board game; bake cookies; take a walk around the block; do something with her. You can take 30 minutes out of your day to do something with her.
Tell her that once the activity has ended, you need to go on and do your homework, or chores, or whatever you have on your schedule.
If she bugs you after you have spent time with her, just tell her that you are able to spend 30 minutes with her each day (or whatever works) and then after that, you need to work on your stuff. Ask her to think of things that she would like the two of you to do together and have her make a list. That way, she has a list and each time you guys do an activity together, she can cross it off.
Third, it's normal for siblings to get on each other's nerves. That's just part of life and part of being in a family.