Boyfriend problems, I need some serious advise.?

Okay, well me and my boyfriend have been together for 6 months. We have a healthy and happy relationship. But we have a big problem here, okay bottom line me and him had protected , the condom didn't slip or anything. But my period is usually regular and it is 4 days late. Because my period is usually regular, I've been stressing out and talking to him about it, it makes me feel so much better when I have somebody to vent to, I think this sounds pretty normal for me to vent to my own boyfriend about being stressed right? Well he always ures me it is ok and that it's fine, and it helps a lot. Okay but he tells his sister about how I'm a little paranoid and that I'm getting worried everyday and she text messages me "chill the **** out your not pregnant stop bothering him about it, even if you are it's your fault too and my period is 2 weeks late and im ******* fine" and yes she over-cussed in those messages, so I took it to heart. Why would he tell her about this? Why would she message me so rudely like this is any of her business anyways? He says he is sorry and that she went too far and that he won't tell her anything. She apologized and said she didn't mean it so harshly. If she didn't mean it like that why did she cuss so much? I feel like I can't even talk this out because he will of course tell his sister. I'm supposed to come over tomorrow and his sister is actually usually nice to me we were "supposed to be" friends, I feel like being really y to her tomorrow because of what she did but I'm not sure if it will help anything. How do I deal with this? Everything seems to be falling apart, how do I talk to him and try to trust him with my feelings again if he tells his sister everything? He is upset and wants to talk but I'm afraid to. Let alone I'm nervous about my period still being late and I took a test yesterday and it said negative. I'm so confuse. Please help