Well, It looks to me like they just got lazy and reused Dracula's speech in Richter's end for Maria as well. So, that might not actually be true that the Renards are related to the Belmonts.
Maria is related to the Belmonts because:
1. It says so in the Rondo manual.
2. Dracula says so at the end of the game.
3. Maria says so if and when she rescues Annette.
In what capacity she and the Renards are related to the Belmonts is never truely explored though. It could be a branch going back as far as Trevor as far as anyone knows. But the relation is there.
Besides that would be a little weird with Richter and Annette.
Not at all. Because Annette and Maria are not related by blood. That was a combination of fan invention and possibly some misdirection imposed by Dracula XX (once again, I hate that game). Too many fan sites have stated this. And its leading to some of you putting it in your family trees.
Technically Annette was never given a last name. She's not important enough to rate one. She serves as the Damsel in Distress, and a love interest for Richter so that the Belmont genes can be passed on. Nothing more.
Also, Simon obviously had to screw around with somebody, so Serena is a good enough place holder even though that game isn't canon.
- John