Yes, I would expect them to be honest, but I think you also need to be conscious of how trauma affects people and how difficult it can be to 'be normal' when actually your family has screwed you up quite a bit. I've been through this twice now - in my family and in dh's family - in both cases the 'parent' was a stepfather, not our biological fathers.
It's absolutely horrific. In dh's case, his family massively minimised everything that happened. To the extent that we have now become the black sheep of the family. It took a long time actually for dh to even be able to talk to me about it (he wasn't the child who was abused, it was a child in stepfather's own family, but dh did experience a lot of childhood trauma that wasn't sexual abuse per se). Dysfunctional families can do a lot of work to normalise abuse. And dh found the idea of speaking out about it to be a 'betrayal' of his family, who are really quite toxic, but his relationship with them is really not normal, because of what he went through as a child.
Yes, I would expect a healthy adult to be upfront about it (as I have been when it happened in my family), but it isn't always that easy for everyone and I think you have to be conscious of that. Dh, with support, and a bit of an outside view on his family (he completely didn't realise that not all families normalise this sort of bullshit), was able to understand how horrible this all was and was very supportive of us cutting ties with this family member when it did finally all come out. But it took a lot of emotional work on his part to deal with his own stuff around it first. Fwiw, the rest of the family has continued to support the abuser and we are very much the black sheep because of the boundaries we've set.
But no, to answer your last question, I gave dh an ultimatum, either you accept that this is not normal in healthy families to support someone who abuses children and you come with me into a new life where we cut off ties with this person, or our marriage will be over and I will take our dc far, far away to protect them.