thank you Carlos. Here I mean my sister only helps paying part of my tuition fee so " in " is the right choice here, isn't it?

It makes a difference whether your sister pays all, or only a part, of the tuition fee. If she provides support "in paying my tuition fee," that indicates that she helps by paying part of it. If she provides support "by paying my tuition fee," then she is paying all of it. "In" suggests that she participates with you in the paying, whereas "by" suggests that she does it all herself. "I needed help in getting my car out of the ditch" means that I needed someone to participate with me in getting the car out. "My brother helped me by getting my car out of the ditch" means that my brother did it all himself.

thank you a lot for your help :D

I would use 'by.' My sister provides support by paying my tuition fee. This also works: My sister provides support in the form of tuition payment.

I would recommend using by or with instead.