If you’re preparing Moms income tax; her gifts, payments, rent, and living expense or contributions to household expenses don’t  really matter…. 

Arms length End of life caregiving/nursing home etc. expenses can be 3-4 times as much …… more importantly moms in a place she is probably most comfortable and surrounded with family….Bobs right about sibblings or other heirs…. They should be on board with whatever arrangement but that’s not really your concern….

worst case is file an extension