The reality of it is they aren't really cousins although they were raised as such nor is Mac their bio father.
Felicia's both Maxie and Georgie mother, and Frisco is their Bio father. Felicia became pregnant with George when Frisco came back for a short visit when Maxie was getting the heart of of cousin BJ who was killed in a tragic school bus accident. The actress that plays Felicia was pregnant in real life at that time and they wrote the pregnancy into the story line.
In real life the actress that plays Felicia and the actor that played Frisco are married.
When Frisco left, Felicia and Mac became involved and married. Max pretty much raised the girls as he's the only Father they ever knen. I don't think Georgie has ever met her Frisco father. Maxie I'm sure has which is why George refers to Max as Dad yet, Maxie calls him Uncle Max.
Now as far as Lucas, he is the adopted child of Tony Jones and Bobbie. I'm blanking on his real mother's name but his father was thought for awhile to be Robert Scorpio who is Max's brother. It was never proven 100% that Robert wasn't Lucas bio dad.
While all these kids were raised in the same house like cousin's they are really not related so any releationship could fly.