Rumlow works with Steve's sister instead of Steve. He befriends and manipulates her into introducing them because (duh!) she has a really hot brother. Steve and Rumlow hit it off and then Rumlow does horrible/ bad/ dirty/ wrong things to him that Steve loves (in and out of the bedroom, however your beautifully trashy little heart desires) but Steve's sister is all guilty because she thinks she's gotten him into an abusive relationship.