My father had strokes multiple times and is under my care. Sister won't let my mom come visit my father. Any suggestions?

I do allow my siblings to come, my problem is my sister that doesn’t let my mom to come to visit my father. Everythings because of something with me that I try to figure out. I’m the oldest male in the family and I have my illness I’m disabled... I need help, everything I got it’s no from my siblings but they want me to do whatever they say. Coming back to my mom, when she comes now in winter they make her walk long distance with the cold weather and wind that makes your hand or body almost frozen in seconds. My question is what I can do to protect my mother from my sister and take her with my father, and stop my siblings in good ways from telling me what to do.