Or was it Heather? There was sex noises from the room before it happened. Did she find Tommy with someone else and decided to take revenge out of anger but then regretted it?
Heather? No, I don't think so.
A banal revenge for adultery, of which there were many, is too weak a plot device.
Why do I think that Jill's father is behind it all...
At the beginning of the game, we see Jill explaining some rules to us.
This suggests that the author singles her out from all the main girls.
Why does he do this?
Probably because she is a kind of key character in the story, besides MC, and perhaps she's part the culmination of the story. We all see Jill as a well-mannered, kind-hearted girl, who nevertheless firmly adheres to her moral principles.
What might her reaction be if she finds out that her parents, who have been an example for her all her life, are not as noble and decent as she thought of them?
This could be a good dramatic turn in this story.