The most people that will take advantage of you are your relatives, mostly siblings. They feel they have the right and should do whatever they like. The guy is 25 years already and shouldn't be told to contribute or support his sister. Being a Christian doesn't mean we should take nonsense, too. Even the Bible said we should be wise and careful, too.

If two people are living in a house, I guess both should contribute except if there has been an agreement whatsoever between them. The lady should take her decision which would give her peace of mind and not trying to fight it.