So your sister has 5 children under 10, one with complicated needs, a husband with hobbies, who avoids parenting, so she guilts her sister who works full time, with her own children, to give her a break.....
I think your sister is a CF and she is using you.
In fact if you need to go Low Contact, then do.
You are being used by her and her husband.
She needs to sort her husband out.
I agree with those who suggest you tell her, you are not able to this.
But perhaps you will have to pull back.
I would be someone who would want to help too, but being used like that would not be on.
You have a fulltime job and need your downtime.
She wants to increase how often you babysit.....I bet she does!!
She clearly thinks you are a bit of a mug OP.
She doesn't have the luxury of refusing outside help...
You have been very generous already....time to nip this in the bud.