Look into filing a guardianship proceeding.
Schedule a meeting with local guardianship attorney.
The answer given does not imply that an attorney-client relationship has been established and your best course of action is to have legal representation in this matter.
I changed your practice area to Conservatorship. You should immediately have a consultation with an attorney who does conservatorships. A conservatorship is where you are looking to control an adult. A guardianship is asserting control over a minor child. Good luck!
I disagree with the other two attorney answers. Conservatorship/guardianship is not appropriate where an individual can understand important issues and make decisions for herself. If you sister is aware and not significantly cognitively impaired (i.e. has capacity), your sister has the unequivocal right to make her own decisions and have them govern her care. If your sister has capacity, she also can appoint you and/or your mom or others to be surrogate decision makers through financial power of attorney and advance directive for health care. For instance, if your sister does quality POA and AD for HC appointing you, then you rather than the children would manage finances and care unless and until your sister objects. If your sister lacks capacity, then guardianship/conservatorship would be appropriate. An elder law attorney possibly with medical consult can advise on capacity. Bottom Line, you should consult a local elder law attorney ASAP. Lawrence Friedman, Bridgewater, NJ. Certified as an Elder Law Attorney by the ABA approved National Elder Law Foundation, former Chair NJ State Bar Association Elder and Disabilities Law Section, Member Board of Consultors of NJSBA Real Property, Trusts & Estates Law Section, Vice Chair Special Needs Law Section of National Academy of Elder Law Attorneys, and Master of Laws (L.L.M.) in Taxation from N.Y.U. School of Law.