How do I defuse this situation between my sister and myself concerning visitation of my 89 yr.old Mom?

I built an apartment for my mother because she had fallen an had to have surgery because of Brain Bleed. My sister talked my mother out of moving in when completed. my mother went to live with her instead. slowly over the years she has gained full control over my mother's finances. My sister slowly decreased my visits with my mother and no its at a point where there no communication concerning my mothers health, medications, plans. she dose not answer my phone calls and constantly changes the dates of visits. Finally after weeks of changed dates I was able to visit against my sisters wishes but she could no longer deny my and my sons visit. when we arrived she told me she was leaving and didn't know when she would be back. when I objected because of lack of medical knowledge concerning my mother there was an all out verbal attack and shouting from both her and my niece. She was yelling "I am in control not you!" I have no idea what she is talking about? My Son is going in for extensive surgery in two days, until he recovers I am unable to even try to communicate with her to try to smooth things over. I am 12 years older than her (68Yrs) and become extremely upset dizzy and short of breath with the screaming. She has taken to defaming me on her face book page to rallying family and friends on her side with lies.