Nothing to worry. Your sister can gladly make an account,but there are some rules you need to follow.
1)You cannot vote up the other sibling. This can easily end up going into the realm of spam up-voting. You might feel more obligated to vote up your brother or sister. So just don't do it.
2)Don't answer your sibling's question. YOU CAN JUST TALK TO EACH OTHER IF ONE OF YOU KNOWS THE ANSWER! Using the Pokebase is not needed if one of you knows the answer and will only be considered point grubbing.
3)This only applies if your sibling uses the same router as you. Also, if you are related and live in different households 1 and 2 still apply.
Just say in the link that your sister(Sister's username) has made an account.
Hope this helps!