Those are questions you need to ask those programs ... do they need to know her income level, living arrangements or who supports her ?
We answer income tax questions here, but we cannot provide answers to questions regarding public assistance programs, SNAP, etc. etc. We can ask if you are truly providing over half of your sister-in-law's support in order to be able to claim her as your dependent. And we can point out to you that claiming her as a dependent does not get you much---all you "get" is a $500 credit for other dependents if you can claim her. Almost certainly her benefits would not increase as a result of being claimed by someone who says they provide over half her support; as for whether they could be reduced, you or she would have to ask SNAP or LIHEAP about that.