I believe you have posted here before with a very similar fact pattern. This is a complicated and difficult situation and you simply cannot get the advice you need through a free online forum. You need to consult with an attorney in your area, and probably other mental health professionals as well. From your latest description, it sounds as if your mother at least may be completely aware of what is going on and is actively choosing to support your sister and her grandchildren. A professional can help you determine if this is a rational choice your mother is making, albeit one you don't agree with, or whether your mother has been unduly influenced or coerced by your sister. (Ex. is your mother supporting and "hiding" your sister because this is her child and she would do anything for her, or is it because she is afraid of your sister or afraid your sister will stop providing necessary care for her. Is your mother mentally competent to make these choices? ) An additional issue may be whether your mother and sister are taking advantage of your father if your mother is actively allowing this "fleecing" to occur without your father's knowledge or consent. (It is not clear from your answer if he is able to understand what is going on.)
This answer provides general legal information and should not be construed as legal advice to be applied to any specific factual situation. It is not intended to create and does not create an attorney-client relationship. The attorney writing this post is licensed in Texas and Washington only and the laws of your jurisdiction may differ.