Our family has run the gamut, on this subject!
When DH's ex had custody of their son, illness (real or pretend) was one of many excuses she used for denying visitation. Obviously, it reduced DSS's time with DH, but it also sent DSS the subtle yet unmistakable message that only Mommy really nurtured and took care of him. She was his only real parent and he should only feel "at home" with her. Daddy was only a person he visited...and you don't leave home to visit people, when you're sick!
That's why many states have rules meant to keep CPs from doing what DSS's mom did - emphasizing that visitation is parenting time and parenting means far more than taking your kids to the park every other weekend. It includes taking care of them when they're sick. If you think about it:
>> If his daughters lived with you guys - or if they were biologically both of yours - you wouldn't have the option of sending them somewhere else when they're sick, to protect yourselves or the other kids in your home who are healthy.
>> If his daughters get sick over the weekend, when they're with you, you guys don't keep them, to prevent exposing their Mom and people in her household.
Pat yourself on the back. You chose a husband who cares more about being a full-service parent to his daughters, than he does about avoiding the inconvenient parts of parenting! In the grand scheme of things, that's the most important thing to notice and remember.
That said, here's the flip side of my family's experience: my ex, his wife and their two little kids live nearby and DH and I get along with them very well. When my sons (with my ex) have minor illnesses (the kinds of things you expect to pass around in a family), they have P/T with their Dad, as usual. But if they catch something that could make their little sibs at Dad's house terribly ill - or if I know their Dad and step-Mom have some big event coming up and can't afford to get sick - or if I know step-Mom's elderly grandmother will be visiting them (and she doesn't need to catch what the twins have), I keep them home.
Divorce isn't ideal. But the ugly truth is, it creates opportunites you don't have in nuclear families! Free child care, if you schedule your personal life around your ex's visitation. And, when the kids are sick, since the parents don't live together, they don't both have to get exposed. It just requires consideration, respect and communication, to take advantage of that convenience without taking advantage of the NCP by denying them P/T.
If there's generally good communication and respect btwn. your DH and his ex, he needs to tell her she should've handled the pneumonia differently. That really was inconsiderate, with a baby in your house and guests over, for your son's birthday. But if communication is generally bad with Mom - if she's the kind of person who might like opportunities to deny P/T - then your DH is right to strictly exercise his P/T, sick or healthy, and not give Mom any excuse to interfere with it.
The one thing that's not up for debate is that, if the girls were on medication, Mom should have sent it. That's a clear part of our state's P/T guidelines. You might look it up and see if it's referenced in yours. That's simply neglectful.