I'm not 100% but I think Marks wife is married to Buster (Brendans son) in real life aswell as all the other relationships mentioned above
That's correct. Buster and Betty are married and their real life son plays Bono.
Cathy = his real life wife
Maria = his real life daughter
Buster = his real life son
Bono = his real life grandson
Dermot = his son's lifelong best friend
Winnie = his real life sister
Betty = his son's (Buster) real life wife
Trevor the missionary = his daughter's (Maria) real life husband (and producer of the theatre shows)
Sharon = wife's (Cathy) real life sister
And Rory is a long time friend, and agent
Mark and Grandad have both been with Brendan O'Carroll for about 20 years.