Is it not Lisa who wrote the letter? David is worried about her parents liking him or not and Lisa comforts him by telling him of their love poems. As shown in the letters underneath, Elizabeth and Frank are the parents of David's spouse (Lisa) because it's a love poem as she mentioned earlier. ALTHOUGH.. David can't be Richard and Janet's child as they only had girls. If you try to switch them so Lisa and Mary are "the girls" in the cassette tape then Lisa's parents are no longer correct. Lisa is Sophie's mother and assuming by how new the "Oh David," letter's paper is David should be in the second row from the bottom and the only open spot is our father.
[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!