Aunty is not being manipulated I think, I think she wants it bad but is in denial because of her moral issues and trying to deny her gayness too.
When Satoko incited her to a bath she accepted even tho she thought she shouldn't accept her wishes and rationalized it by saying "if it's just a bath it's ok" but immediately after she started imagining her niece fucking her so she knew where this was going.
When they were taking her clothes off she was checking her niece's body too and was pleased with what she saw.