My little cousin was very tiny for her age
Why do  you think she is tiny for her age?
She looks smaller than other people her age.
If she is smaller than others then she might be feeling self-conscious about it. It might help to build her confidence up. Spending time with family builds confidence for all of its members.
I don't want to. She was dumb and annoying
Saying things like that might her feelings. It would be good to spend some more time with her so that you can see her good points.