Problem

I can see what you mean, you're going to need a better one than that. You could get them to try use it and they might see that it is indeed too old and that you need a new one. If they use that argument then try tell them that you aren't going to be out all the time so when you're at home you need a decent computer. I don't know how old your sister is but she may have her own things to do so maybe tell your parents that you need one for when she is busy. To get your parents to buy something you want is difficult because it depends on what it is. If it is something cheap then it shouldn't be too hard if it expensive, like a computer, then try coming up with key points as to why you need it and then having a discussion with them about it.

You really shouldn't beg for your computer. If you want a new computer and well lets just say you are too young to get a job atm and your parents can afford it (Ask them if they have enough in there budget to pay for one) then you could perhaps work a deal out with them like do chores and earn some money and see if they will pay half. (obviously they are still paying for the whole thing) but you are working hard to get it. Also tell them that you think girls are iky and you dont like playing with them.