My Sister got angry with me.

My sister is a lesbian and had a partner who was a bi-sexual than just a lesbian. So, her partner got attracted to me and she joined me in my bed several times. My sister and her partner live with me, I'm the older and have a great job house provided by my employer. So, I have free rent and 3 bedroom 1-bathroom home to live in. Hence my sister deciding to live with me with her partner. I don't know if she knew her partner was bi-sexual before she caught us fucking. But after she did, she got very upset and angry at me and not her partner as I must have seduced her. Her partner told her to relax and chill out. It took a week for my sister to talk to me again, but after that she had agreed with her partner that she should share with me. As her partner liked cock and tongue just the same. But there was a slight change, I could still fuck her partner, but she would also join in a threesome with her partner and me. But I was only to fuck her partner and that how it is now. Except her partner is pregnant to me, it was agreed she get pregnant as my sister is marrying her partner next month and they want to start a family straight away.