“Yichud is not always an issue with adoption. When we adopted, we were told it was not an issue for us. Unfortunately, I didn’t ask for an explanation, and I can no longer ask that posek for one.”
I think I once read an article on the topic. It was around 30 years ago, so I can’t say I remember precisely. But I vaguely remember a halachic discussion about whether or not tznius issues like yichud and negiah apply to adopted kids. It had to do with whether or not the reason that these things don’t apply to family members has to do with the fact that you view family members differently because they are biologically related to you or because you grew up with them. I think there were opinions that it is because of the second reason, and t/f these issues don’t apply to adopted kids.
Obviously, no one should posken based on this. Especially since it may be a minority opinion or it may not be a valid opinion at all.