[SPOILERS]
You have a lot of things correct here! Just some small stuff that is mixing everything else up
All of these things you have are correct:
1. The parents of the person who writes the "Oh David" letter must be Frank and Elizabeth, since they wrote the love poems
2. Richard and his spouse only had girls, as discovered in the cassette tapes.
3. Lisa is Sophie's mother, as discovered in the mp3.
I think the major thing that is mixing you up is the placement of Frank and Elizabeth. The letters they wrote to one another are meant to look the oldest, so they're meant to be much higher up in the family tree. I think from there you're on the right track to sort everyone else out.
Thank you for taking your time to play the game, and let me know if you need anything else clarified!