YANBU - it sounds too much for you to HAVE to take on tbh
You dont have to. Repeat with me - I dont have to do this!!
They are her kids, they are VERY young...overnight stays can be tricky, what if something happened - to you if you get ill, your DC or her DC?
Stay back from it, gently say you cant commit to it and believe me, ignore her calls and she will HAVE to find someone else
Her DH needs to step up to the mark here...
Getting soooo fed up reading about ''friends" shunting their kids care onto friends - happened to me recently, what started as an irregular playdate with my DS and another boy, suddenly turned into the boy's mother arranging for her other older 2 DD to come to play also. And stay for tea. THREE TIMES A WEEK. Oh and i had to drive them everywhere for their extra curriculars...
You are responsible for YOUR family - she is responsible for HERS
There are only so many favours you can do for someone - dont worry, the hospital will advise her
stay strong
your gut instinct to not do it sounds right thing to do