Why not write your sister a letter? Don't even mention that her husband excluded you from the visit.

Instead, focus on her. Just tell her that you love her, you're praying for her health and that, no matter what, you will always be there for her.

I find that a dose of positive energy and kindness are the best response to negative, nasty people. Don't let your ignorant, insensitive brother-in-law drag you down to his level.