I appreciate everyone's input, thank you. I know I chose the wrong wording when I said "opt out" that wasn't the way I meant it.
I would've appreciated a head's up on the pneumonia (that spread like wild fire through their baby-sitter's house too, so yes very contagious) I believe that is common courtesy.
When my DS is sick, I make him stay in bed just like I did/do when I'm very sick. He is also very mindful of covering his cough/sneeze and washing his hands. That isn't really important at their Mom's house or at the baby-sitter's house. That's how I deal with it when my DS is sick. We do have motrin/tylenol/dylsum, we don't have their antibiotics though.
When my DS was seeing his biological father still I would warn him when he was sick and offer to keep him home.
She is not the type of person who would deny parenting time, she jumps at any opportunity to have a child free weekend. That is why our every other weekend schedule has now turned into almost every weekend.
Aren't you happy that your husband and his children get to see each other more often? I can't imagine only seeing my kids twice a month.