It's not overly explicit that they're just friends, seeing as they have photos of their kids together from years ago. They may not have been living together, but the idea of them being romantic isn't exactly far fetched, seeing as Debbie took the MC in and treated him like family.
- Frank would've slept in the same bed as Debbie in that very same house, even if not always.
- There was only one car, because Debbie was playing the role of the housewife, even though they weren't married.
- Again Debbie was the "housewife" while Frank was the man of the house.
- Debbie took the loss of Frank hard, because she was romantically involved with him, even though they weren't married.
- Jenny felt he was like a father figure, even though they weren't related.
- Is it implied that they shared a bank account? I'm not sure on that one. If they
did share a bank account, perhaps it was because they were playing house, while not married. The only other financial connection I recall, is the debt that's owed to the Russian mob by Frank. The mob isn't concerned whether or not they were married.
- Why do they look alike? Look up Twin Strangers
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That's always a possibility.
- Erik and Tammy look like family because of the pre Patreon storyline?
Those are just a few answers off the top of my head.