The Sister-in-Law (1974)

A man becomes involved with his brother's wife and mistress in The Sister-in-Law (1974), an appropriately sleazy slice of '70s softcore trash. Robert (John Savage) has just returned home from an unnamed "long trip"; when he gets there, he finds out that his brother Edward (Will MacMillan) has taken a mistress (Meridith Baer), but hasn't quite left his wife (Anne Saxon) either. So Robert does what any red-blooded American male would do: he starts screwing around with both of them. You'll feel like a member of the family when you get a glimpse of Meridith's mams screwing Robert in the woods and Anne fully nude as she brushes her hair 3 minutes in and again drying herself off at the 14-minute mark. Talk about Mam-ily ties!