His Ex🔫coming back as a🔪SISTER!!😡

San • A 22 years old with a loving bf that's 5 years older. Interracial couple. Love him💕
I know this sound crazy but my BF's first love(Megan) who happened to be my BF's best friend's sister. 
So. My bf(Stephen) and his bff(Jerod) were so close as brother and their bond were inseparable promised to each other if anything ever happened they would take care of the others family like their own. A year ago, Jerod passed away due to a car accident. Stephen were dating a girl named Carolyn at the time she was super jealous, he didn't wanted to upset her so he didn't comfort Megan like he thought he should been there for her.... Months later, I come in the picture. While working on our differences in culture and religiously between us, our relationship is at it best! But recently, Megan been having bad dreams, she can't shake off her feelings that she still moaning for her late brother and her BF doesn't really help, instead try to distract her(according to my BF anyway). Stephen feels the need to comfort her, by talking, invite to family dinner, and last night, his birthday party! I meet her for the first time last night, I liked her, and I thought she did vice Versa. Nothing was wrong until Stephen(as a host) walk her out to her car while everyone still in the house...FOR A LONGGGG TIME. As a gf, I'm curious! So I got upstairs and peak out the window to see what they were up to. Later, when she were gone, Stephen confronted me that they saw me stalked them(Megan pointed out). He's mad at me for not trusting him. I told him the whole thing made me u comfortable, I trust him but not her! (She had history of being a cheater). So he said I should of trust him, and because of Carolyn he felt pathetic in the past that he couldn't help her. That she's so important as a sister that if any girl including me trying to stop him helping her, it's over! Basically take it or leave it. I told him I felt bad for looking out at them and I wanted to make up to her by getting friendly and if he could hook me up, and he was like ,"I'm not sure she wants to talk to you, cause she was excited to come see you, and you acted like that. She felt out of place in the first place last night cause I didn't talk to her". I told him that he's defending her, and he was like "of course! Cause you're wrong"! 
Idk, im hurt and lost and confuse. Idk what to do.....😰