@boomerang,
Aunt needs to take some responsibility for what she has done. If she keeps disrupting Mo's life with this stuff, maybe she needs to only have supervised visits with Mo. It isn't just Mo, it is your family and the new family of Mo's other mom that she has so thoughtlessly disturbed.
She needs to help clean up the mess she has made., especially if OM does not want contact with Mo right now.
We have a similar situation in our family. I am aware (have been since i was a pre-teen about Mo's age) that I have an older brother from a previous marriage. My mom gave up custody as part of their divorce when he was a toddler. She, nor we, have ever had contact with him. My younger brother and I always kept aware of where they were living and wanted to meet him.
Mom has always asked us not to contact him because she doesn't know if he has ever been told about her since his father remarried soon after their divorce and no one has ever attempted contact with her.
A few years ago I became aware of him being on Facebook and have wanted to contact him, tell him about mom and show him all the photos we have of him. I thought it would be the last chance for he and mom to reconnect before her dementia worsened. I asked her about it, showed her his adult age photos on Facebook, and asked if she was interested in contacting him. She decided not to disturb things after all this time and let things be.
I still go to his Facebook page to see what he is up to. The family resemblance is uncanny. I haven't friended him or made any contact, but I think about him now and then, especially when I read these type of posts about your family.
I don't know if he was ever told about mom or if he knows he has a half sister and brother. I wish there was a way to find out. I suspect he does not since there have never been any contact attempts that we are aware of.
I'm not sure which is worse, knowing and knowing no contact effort has been made, or having contact effort rejected, or knowing but not knowing if they have been told.
Sometimes it sucks how complicated families can be. I hope your two families are able to navigate the rough waters and can satisfy everyone's wishes, whatever they may be.