It's very simple Sue is suppose to be Randall's mother, adoptive, step or actual, or caretaker if you really have a problem with incest. But she needs to have been the one to have raised Randall.
It's really the only way the narrative in general and Sue's reluctance to mess around with him makes sense.
She is likely suppose to be 18 years or so older than him, so between like 36-40.
If she's not his biological mom then those ages can skew younger
Randall is around a freshman in College so like 18-19 and again if not biologically related he could be a bit older
the time-line is something like this: