I took care of my 93 year old mother in her house for 13 years. My oldest brother, who is power of attorney , came from out of state sold her house and had my other brother take my mother you a retirement community 200 miles from me. I would visit every other week. After a year and a half my brother and sister in law moved my mother into their house against my mother's wishes. That was 8 months ago. When I went to visit my mother for a few days at Christmas time, while my brother was out of state working, my sister in law was extremely rude to me. Before Christmas came she called the police and had me removed from the house for no other reason than she didn't want me there. I didn't get to say bye to my mother and haven't seen her since. I have talked to her on the phone several times and she always says,'when are you going to come see me?' Over and over. A month ago I told my mother what my sister in law did in December and now my sister in law will not allow me to talk to or visit my mother. Can she do this? It makes my mother and myself very unhappy.
Write a lovely card to your sister-in-law and tell her you'll be sending newsy cards to your mom and hope/pray they'll give them to her. Stay in touch that way. In a few months, call your BROTHER and ask if you can visit a few days staying at a motel. They are under no obligation to open their home to you. Your SIL probably feels she has quite enough on her plate without that.
I suspect your relationship with these people has been broken for some time. Now is probably not the time to fix it. Good luck to you...
I'm not sure legally, that she can prevent you from seeing your mother unless she has guardianship. However, the fact that your mother is in their house changes that up a little, because she has the right to say who is in their house. When you visited in December, did you invite yourself or did they invite you. Did you stay at the house, or were you in a motel. Did you eat with them or offer to provide a meal? I don't know your sister in law and I don't know you, but I do know that after a couple of days of company, no matter how much I love them, I'm ready for them to leave. Especially around the Christmas holidays when things are so hectic anyway. You might give your sister in law some time to get over that, and then approach her with a calm attitude and ask her if you can come visit. That you miss your mother and would like to see all of them. Maybe enough time has passed that she will be in a different frame of mind. Good luck!
Can your mother leave the house at all? Perhaps you can arrange (somehow) to visit with her at a different location.