get a STD from your sister, or not getting anything from your mother but still doing her. (and neither isn't a option)
Think about it, the STD would suck, but the mother option would probably be more awkward... unless your closer to your sister... there's no real right or wrong... well there's no real right answer, there are many wrong answers though
Think about it, the STD would suck, but the mother option would probably be more awkward... unless your closer to your sister... there's no real right or wrong... well there's no real right answer, there are many wrong answers though