Ready to slap my MIL lol

Not sure if she knows the details of your surgery and what you're going though and not sure what your relationship was like with her previously but sounds like sabotage to me, especially since she didn't even stay. Trying not to judge but why would the sister bring out a sick child? Anyway, my hubby and I came up with a good plan years ago when we got married - he deals with his family and I deal with mine. If I have an issue with his side, I go to him and hopefully he deals with it (doesn't always happen though) and visa versa. Would have likely been easier on you if he said no to them instead of you.  

Try to keep your head up and good luck with your surgery - I'm sure you'll be healthy and be on the other side of this in no time!