How much of an accent is family vs wider environment?

So this is the kind of thing that floats across my brain when I cannot sleep, but how much of one's accent is ones immediate family vs their peers/environment? So for some context, I am from TN, whole family is from TN and I sound like it. My SO though, while we were both born and raised in the same town, he doesn't sound like it and his accent is hard to place? His mom is from Michigan (and sounds like it), his dad is a bit more interesting. He's from Indiana, but HIS dad (my SO's grandfather) is from the south and his mom (SO's grandmother) is from England (I never met either of these people and have no clue what they sound like) and his accent is also hard for me to place but different from my SO's.

So what I am wondering is, has anyone looked at or know anything about how one's immediate family vs overall environment impact accents? Like what would our future child sound like if we moved to say London? Yes people they saw on TV or friends at school would have British accents, but they grew up in my house listening to me all day long. So I'm just REALLY curious about this.

I appreciate any answers to my late night query.