So, to summarise:
"She always prioritised his stepdad during his childhood, even when [his stepdad] was abusive to him." "He actually lived with his auntie for a time as a teenager" which suggests living with his mother&stepdad was either intolerable for him, or, his stepdad wanted him out and his mother was again prioritising her husband over her son. He presumably moved back at some point but his home life was again unsatisfactory, which is "the reason why we moved in together so early on." Despite not providing a loving home for her son, "They feel like I took him away from them". (Because, there is a type of person who, even when they don't want 'something', get very possessive should anyone else show interest in the 'something'.)
Rather than drawing a line and forming a new adult-to-adult relationship with her son and you, the woman he lived with, his mother instead pushed the two of you away by "always excluded[ing you] during Christmas and other occasions" and never being particularly welcoming.
On top of all that "His mum told him she downed a bottle of vodka when she found out she’s having him" , effectively telling him she tried to abort him. What a lovely thing to tell your child!
Did I read your posts right, @JanineR ?
Honestly - fuck her. She was an appalling mother then and she remains an appalling mother now. She treats her son and his fiancee like this and then expects to be prioritised? Pfft! I'm surprised your fiance talks to any of the family apart from his beloved aunt at all., never mind involves them in his wedding. It would probably be best for him to put some distance between himself and his birth family, who give him nothing but grief.