Did you check them out in Family ties as well as sim relations?
Yep, done that. His mother is Bella, as it always was. Dina is no longer shown as married to Mortimer, I removed that tie, and Mortimer isn't shown as married to Dina either. The only connection between Dina & Alexander is that her children are his half-siblings, although that shouldn't make any difference because at the end of the day, there's no blood relationship between Dina & Alexander. I had the same problem in my original Pleasantview - Amin Sims had 2 bolts for his stepdaughter, who wasn't shown as family (even though he'd brought her up from being a child), but no romantic interactions were open to them. Even worse was Desdemona Dreamer, who had 3 bolts for Jonathan Burb but they couldn't interact romantically because he was married to her niece. The ridiculous thing was, Jonathan was related to his wife by blood (he was John & Jennifer's son and he married Lilith & Dirk's daughter, who was Jennifer's great-niece) but the game didn't acknowledge that relationship.