Like it or not chances are he lied to you just as easily as he lied to his wife.
Like it or not if his marriage was as unhappy as he's claimed to you
A there'd have been other symptoms which sound unlikely
B he'd likely have left her before he met you and CERTAINLY would have left more easily once he HAD met you. Especially if you were the great love of his life you claim to be.
I've also got friends now in their 30's & 40's that were the children of the earlier marriage. They've certainly had trust issues, mental health difficulties, difficulty being faithful themselves, maintaining friendships... Those marriages broke up when they were in their teens. So the effects certainly lasted more than 5 years for them.
Our situations may not be directly comparable in some ways. But don't kid yourself that just because he sees his child regularly that he didn't abandon her, he did at least emotionally, and in terms of no longer living with her, do you think that she doesn't/won't in the future question why he couldn't be faithful to her mother for HER sake, question why YOU thought it remotely OK to have an affair with someone who was not only a husband but a FATHER too? question if you even wanted her around at all...? Why are you assuming dsd doesn't already know about the affair? Kids aren't stupid. BUT if that's true it rather contradicts your claim of its having no effect on her, if she doesn't know how can it? The affair side I mean. The divorce will still have had an effect.
Did he leave or did she kick him out?
Digitally it MAY be true in your case but I think it more likely they don't want to hurt your feelings, as you are not the guilty party.
There are marriages resulting from affairs which succeed (more don't, the more often you've been married the more likely you are to divorce again), but in this case the op has already said he was repeatedly unfaithful in his earlier marriage, that strongly indicates he struggles to stay faithful so unfortunately for ALL the children involved I think this marriage is unlikely to be one of them.