My family is poor so I have to wear my sisters old school uniform and my friends laugh at me, why?
Sorry to hear that you are having to wear old school uniforms.
If it is any consolation this is actually a very common practice and probably some of the friends or people who are laughing about it are secretly doing the same things themselves, they just didn't tell others.
School uniforms are really expensive and it makes sense to recycle perfectly good uniforms from other family members where it is possible to do so.
Rather than let it worry you just get on with it and relax and enjoy your time at school.
There are much more important things such as learning, having friends and fun when at school so just try to view it as a silly little thing that really shouldn't occupy your mind at all and you should be just fine.