You're staying with your sweet big sister and her husband during your divorce and having a really tough go of it. You're sad but horny and feel closer to your big sister than ever, so when she comes to check on you, you ask her to climb into your bed and comfort you the way she did when you were younger. After some sweet gentle sisterly love, she climbs on your cock and grinds nice and slow, until you fill her pussy with your cum, and she realizes she didn't tell you she wasn't on birth control.