Ok, little background. I'm 25, have a good job, live in an apartment with my girlfriend of 5 years. She has a sister(lets call her betty) whom I have an on/off relationship with- we get along, but have never been able to really get a rapport. Her sister is a hardcore feminist who got married to a guy when she was 19 (21 now), who is in the marines (probably for the benefits).

A couple times, when she was drunk, shes displayed some weird behavior towards me- One time, we were at my girlfriends parents house, and she wanted to show me something in her phone. She came over, sat on my lap, and put her arm around me rather than just show it to me like a normal person. I thought this was weird but let it happen because my gf didn't seem fazed by it. Betty also seems to like complimenting my clothing, hair, muscles etc.

Last night, My girlfriend, Betty and I went to a party. She once again complimented something (my jacket) before we left. We go back to girlfreinds parents house to sleep 5 hours later. Before we go to bed, my girlfriend told me that betty told her, when she was really drunk, that she didn't think my girlfriend was happy with me- my girlfriend literally laughed in her face at this- but my question is what the hell is going on here? I can't decipher this at all.