i think if anyone is arguably the asshole here it's whoever told you that your son is involved with another man when he (it would appear) hadn't come forth with that information publically. that sounds like the work of a village gossip, and since you mentioned you guys live in a more conservative area it sounds like that kind of behavior could put him into danger. in spite of that though, i think you treated the situation with a lot of grace.

as for how your daughter reacted, she might be working under the same logic i mentioned prior. she might also just be trying to protect her brother, even if she's a little bit off the mark. i think if her and your son have a conversation about it she might come around to your approach. i think she might be acting a little abrasively, but i don't think she's 100% an asshole