I think it depends a LOT on the family. IN some families yes it would be strange...but in others - nope, it would be normal. In my family there is no way my sisters wouldn't be there night and day if I needed them (and, this is a biggie - I ASKED for their help - have you or dh asked SIL for help) but in dh's family, they are close but tend to do their own thing. They will ask for help for big jobs (doing stuff to the house, etc. for the mom - the boys will all get together, etc.).
One thing for the OP to consider - Has your dh gone to help his sis with anything over the last year(s)? I'm sure she could have used assistance here or there...Maybe not with major surgery but with sickness - bringing her a meal, house things (painting repairs, etc), etc. - We ALL need help once in awhile. And, I'm making an assumption here, since you didn't mention in the post - but it doesn't sound like your SIL has any kids? Or at least not young ones? Just because she doesn't have kids doesn't mean she's readily available and also doesn't mean she hasn't needed help with other things in her life.
If your dh doesn't have the type of relationship with her that he would have gone to help her out with stuff over the years...It's not really that surprising that she wouldn't do so now. NOW, that said, back to my mention of asking for help...People aren't mind-readers - If you would like her help - ASK for it.
I try to think of it from the point of view that not all families are the same and that at least sometimes, sister bonds tend to be a bit stronger (most of my friends it is the daugthers/sisters taking care of things for the elderly parents and organizing family events, etc.) - so I know that my EXTREMELY close relationship with my sisters is not what my dh has in his family. They are different, but close, and I know if I asked for help they would be here.
Best of luck with the recovery.