Heh. I’m sure her insult is unclear (it is a bit even to me), but she is insinuating that his mom slept with his uncle to conceive him, and so his mom would also be… his aunt. Except for not really, because she’d have to marry his uncle in order to be his aunt. but then she’d no longer be his aunt.
Life is so confusing when you’re hurling insults.
Or, more disturbingly, that his mother slept with her brother to conceive him, so that she is his aunt — his father’s sister — as well as his mother.
…and I’m my own grandpa.