It's a huge deal and she very definitely should have checked with you. Absolutely, cannot be argued that it's not something you discuss first.
Many, many, many people couldn't do it, for all sorts of reasons. Your DH doesn't get to decide, of course. He can decide for himself, but not for you.
Anyway, I am wondering, would he be expecting YOU to do the lioness's share of parenting/caring?
Even if not, are you supposed to just smile and accept such an enormous hypothetical alteration to the rest of your life, with zero input from you.
Er, no. Of course not. The will/finances/expectations must and should be discussed and you are an absolutely normal person for wanting to do this.
When my kids were young (they're all in their mid to late 20s now) we named one of my ex husband's brothers and his wife and asked them if that was ok. We explained what we'd want for our kids, and left them financially covered in that event by adding them to the will. We wrote everything down and ran it past them, just in case.
I'd look after any of he kids in our family, if it came to it, but you might not be able to for all kinds of reasons and you might not be the only option either.
It's certainly not your SIL or your husband's right, morally or legally, to try to bulldoze you into something of such magnitude. A conversation MUST be had if she wants to put this huge responsibility on you.
Of course, the chances are it will never happen, so you could just think "Nope" and let it go, knowing you would never agree to it if push came to shove.
More concerning, perhaps, is the way your SIL and husband appear not to care at all about your feelings on the subject.