Our donor is always referred to as a donor...never anything else. We are very thankful for him, but he is just a donor, not a "father".
We are, however, in contact with 2 donor sibling families (3 of the kids). We are FB friends. I love seeing photos of the kids and being able to watch them grow. We know of 4 other families. I have reached out to each of them and they are not interested in contact. We respect that completely and do not contact them. We do at least like to know they exist, if nothing else. We like to know the number. Including my son, we know there are at least 8 kids.
They are his donor siblings. I've never thought of them any other way or called them by any other title. We've never discussed it, but I imagine the other families feel the same. We are just people who happened to use the same donor and our kids share some dna.
It is important to us to have these connections already in place when possible. Our son didn't ask to be donor conceived. We want to make things as easy as possible for him, should he want to pursue relationships later on.
Sometimes, it's weird to think there are people out there, strangers even, who are biologically related to your child. But this is what we chose. We chose to use donors and that comes with it. It is what you make it. For us, it's just a part of life...not a big deal.
But in any case, being curious about donor siblings is completely normal. Not being curious is strange to me, but that's probably normal too. We all feel differently and do what we think is best for our families.
Mom to P (12), J (11), E (6), and awaiting the arrival of our last baby in late December 2017.