Do you think my sister has a logical reason to defend her friend for sleeping with my girlfriend?

I'm 26 year old biual woman and my sister Rachel is 19 and so is her best friend Jenny(she's too). When Jenny's parents kicked her out, my I took her into my home as my "adoptive" little sister in September 2008. Me, my sister and my girlfriend Vickie live together. Two months ago I discovered that Jenny had slept with my girlfriend. So, Rachel and I were devastated so we kicked her out. It took me less than a week to forgive my girlfriend. At the end of the day, butch s like Vickie are like men, men will be men, men will cheat. But Jenny should have known better, I took her in my house as my little sister and this is how she repays me? But I was still nice enough to tell Rachel to forgive Jenny even though I was furious with her. Last week, I finally invited Jenny over and she apologized so I forgave her as well. However today, Rachel and I got into a huge argument and I brought up Jenny's name. Rachel gets all mad because Jenny "had nothing to do with what we were arguing about". Rachel has no loyalty to me as her sister(THE ONE WHO TOOK CARE OF HER SINCE OUR MOM PED AWAY). Rachel actually cried in tears and cursed at me like I'm a piece of garbage because she didn't appreciate the way I was referring to her precious friend Jenny as "that b*tch" "that sl*t" and "that ". What logical reason could my own flesh and blood have to defend her dirty friend who slept with my girlfriend? Do you think I have the right to feel betrayed? Rachel hates Vickie by the way.