It's been a looooong time since I played it, but I seem to recall the aunt actively helping the MC's harem in
The Visit. I think that one had MC's mom and girlfriend, can't remember if there was a sister... but I think the aunt helped you both win over the (very straight laced) mom and get the GF to accept sharing the MC...
As mentioned above, it's a common trope... a lot of incest games use the aunt as the weak point in the family by making her more open minded and wild. But your description really made me think of The Visit in particular...