What if I tithe and neglect my mother?

The best thing you can do is help your family. Widows and Orphans are to be helped first. The other thing is not to judge what others do with their money no matter how badly they behave. That is coveting. If you help your mother then you do not have to answer to God about your own actions. Your sister will have to answer to God about her own actions. Some people just do not understand because they may not really believe in God/Love. God is Love. Do Love unto others as Love would do unto you and live with a clear conscience.