Pregnant Sister

My sister Cassie 24 got pregnant and her then boyfriend cleared out leaving her alone and out of money. I'm David 27 her only sibling and only member of her family that lived close by to her. I'd started a great job after finishing university and had my own place a 2 bed apartment close to my work. So I took Cassie in and at first nothing happened, but Cassie got bored and extra horny as the time passed. I didn't have a girlfriend at the time, but was fucking my supervisor Charlotte 47 married. She was a little overweight and liked cock and her husband was away a great deal and we had just started having sex regularly at work after everyone else had gone home. No relationship just fucking and after getting home one evening after fucking Charlotte. Cassie was very horny and I caught her playing with herself on getting home. I got aroused and we ended up fucking and she moved into my bed after that. Now she has a baby daughter born in December and still sleeps with me still. Quite happy with way things are and nobody knows of our sexual relationship.