Can you describe the bump you experienced on your lip and bit more completely? People have posted pictures here but honestly I’m not sure how to do that. I would encourage you to try harder.
Having asked that, I think transmission in your situation is really unlikely. You being a double negative person (no HSV 1 and 2) might have a bit more dramatic presentation if this is a first infection. Not everyone who gets new herpes has that, however, there is a lot of variation between people on that one. But then we also have your niece who has had something in the corner of her mouth for a month? Most people with new herpes do not have it for a month at a time. But there is condition called angular chelitis that we sometimes see in children and adults that is not caused by herpes, I’m wondering if it could be something like that? Or chapped lips from the cold weather?
Antibody tests for children are NOT accurate so even if you could ask, it wouldn’t be good to ask.
Overall, this doesn’t sound like herpes to me but I understand that you are worried. So do you know that you have neither HSV 1 nor HSV 2 because you have been antibody tested? If yes, then what I would suggest is that you get retested now. What are you – six weeks out now? If you are negative for both still, there is a 70% chance that you will stay that way at this point. Does your boyfriend take suppression? If yes, the likelihood of transmission in this way is additionally lower.