Am I overthinking or am I right?

Mikayla mind has just been driving me crazy...Its telling me that there's a possibility my boyfriend could be secretly in love with his brothers half sister? CRAZY RIGHT?! lol...Backstory, he tried cheating on me like 3 times in the beginning of our relationship and it took months to trust him again, till now....We are going on 11 months, and his brothers half sister is coming into town...Now he says how he thinks of her as a sister..but idk..I didn't think much of it till he just became distant with me and talking to her alot..and like, it doesn't help that shes drop dead gorgeous and insanely smart..I'm just kinda nervous about meeting her, or if I ever will.. I've been bullied and cheated on so my self esteem is at 0%.. What do yall think? if it is nothing any advice on meeting her?