D.P.
Hi W.,
Ain't families grand? NOT! :-)
From your post, I don't get a sense of how far (travel time) your sister and mom) live from you.
Is it possible that your sis is resentful for being the main caregiver for your mother? Maybe she feels overwhelmed and unappreciated? She may think it's easier for you to go there, rather than for her to either bring your mom or come without her, as it may not be good for her to be away from your mom if she needs constant care. Just a thought. Also, maybe she is looking for help from you (watching your mom) so the she gets a break and can do some things of her own--errands/shopping/banking/etc.
That said, if we're not talking about a "hours long" trip here, I think you should continue to go and visit your mom. Whether she/your BIL make you fell welcome or not. After all, your family has a right to see your mom as well. She just happens to live at their house. I would let them know in advance when you will be coming/decide on mutually agreeable times & dates & go so your mom can see your son! You may regret it if you don't. If your mom can manage with a wheelchair, etc., maybe you can still manage getting her over to your house sometimes? Again--not sure whether we're talking "across town" or a 12 hr. drive here....
Now, if the situation is that you don't care to see ANY of them, then that's a choice YOU have to make for yourselves. But I seem to be getting from your post that you feel mainly that your sister is coming between you & your mom. If that's the case, I would try not to let that happen. Kill her with kindness if that's what it takes to work out visits.
Good luck and God bless!