My mum had a baby when she was 16, a little boy. She gave him up for adoption (wasn't given a choice about this by her parents). He contacted her about 15 years ago, which is when I found out. We all met up a few times but then contact fizzled out. Recently he got back in touch wanting to meet up again and due to various logistics it was easiest to meet at my house, so my children (10 and 8) met him, along with his son (he's a single dad). We didn't tell the kids who they really were (my mums choice). The visit went well and my mum is keen to have them come visit again in the new year.
I feel uncomfortable continuing contact and not telling my kids who they really are - he is technically my half brother, so their half uncle and the son is my nephew, their cousin. My mum feels uncomfortable with them knowing because 'they won't understand' but I really don't like family secrets and feel like it needs to come out eventually if we are going to keep meeting up
Would you tell them? How?