An executor has NO rights over the assets of a living person. Also, the fact that your mother has the beginning stages of dementia does not necessarily give anyone control over her assets, not unless (1) she created, while still fully mentally competent, a power of attorney giving a person certain control or authority; or (2) there has been a court proceeding which adjudicated her incompetent and gave some person (a guardian; who may be a child) control over her assets.Without therefore having some legal right, either given by your mother (e.g. POA) or by the courts, the brother and the sister have no right to do what they did, and what they did may be theft (e.g. it could give rise to criminal liability as well as civil liability). And even with authority, the brother and sister would generally be bound to act in the mother's interest, not their own.Note that you and your other siblings do not necessarily have rights her--it's your mother's assets; she is the one who has rights over what is done with them. If she is now incompetent, it may be necessary for one of you to try to get apppointed her guardian and take action on her behalf; or, if not you, get some guardian appointed who will take care of her interests. You should consult with an attorney who specializes in eldar law about what precisely to do here.