I hope you not only give him ethereum but you can also give him more knowledge about crypto so that one days she doesn't fall into hole that can lose a lot of money
I hope he would do that because personally giving her ethereum without educating her would be useless because if you were not going to educate her then better to give her a fiat instead.
She would definitely appreciated it if ever your younger sister had already an idea about it plus advantage of having the thought that the younger sister of yours know how to trade or manage a crypto coin then it was much appreciated.