I guess because they are older (17 and 15 years old) we expect them to understand that a little baby requires more right now than they do. DH has talked to them about this. Just as a 17 month old is old enough to be taught not to stick her fingers in someone's mouth (and we are working on that), they are old enough to understand that a baby requires more time than a teenager does. They also are old enough to understand it won't be this way forever.
This is not the first time they have had a younger sibling. Their mother has a 4 year old and they took a backseat when she was born too.
Babies and toddlers need more hand's on time, they take us up in different ways. Teens need clear emotional commitment. They need dad at events, on the phone and seeing them every week. If they aren't coming to him he needs to be going to them. You say he's seen them for a movie and a pizza. That has been two times since March. Not ok.
The kids don't need to be the mature ones and just understand your stressors. Dad needs to be the mature one and parent his kids. He needs to take his happy butt over to their house and visit, play video games, go on bike rides, take them to dinner and toss the ball around.
I understand your concern about your dd, fine find a new middleground. Dh should have been the one to explain the issue and then come up with a soultion with the kids. That they would have understood. He should have set a clear time line, so that the kids could have understood exactly what he meant by "for now." It is his job to help them to understand and not just throw out general statements.
It would have been easy to say: "Guys, we are having some health issues with dd. She has been sick so much and the more people she sees the more she gets sick. Her immune system is still growing and, right now, it is overwhelmed. She needs some time to get back her strength. So, I want to talk to you guys about how you think we can come up with a plan for me to still see you while she gets better."
Not too hard. That way the kids feel included in the issue with their sister and they don't feel dismissed from the family. They could have worked out a 3-6 week plan. They could have made a list of fun activities to do and set days for dh to visit to do the planned things. You could have made a great fun list of things no one has tried, or things that were extra fun. That way the kids get excited about the events, see that dad is doing everything he can for each member of the family and everyone stays involved. That would have made the part of the plan, not a apart from it.
It's not too late.