My boyfriends brother is uncomfortable around me?

Itsjustmegurl3
I've been in a relationship for 3 years and am in the process of moving in with my boyfriend. I've chatted with his brother on fb messenger for a year or two and had him stay for a week or two last summer. I'm 22, bfs 25, brothers 20. He stayed 2 weekends this summer and won't come back. He told his brother he's uncomfortable around me and he won't tell me why when I asked him. I know he's never had a girlfriend before and this summer some girl used him and he's a lonely kid. I have a lot in common with his brother and I bought him an anime thing and I call him my bro and spend a lot of time with him. My boyfriend encourages me to talk to his brother who is lonely. He's been moody the past weekend or two and I have no clue why he's uncomfortable. Does anyone know what's going on with him possibly? He's supposed to sleepover at my place with my boyfriend for 2 weeks (my bfs planning not mine) and I'm kinda scared now my boyfriend works all day and I would be alone with his brother 24/7. I'm afraid it's gonna be awkward, since he said he's uncomfortable.
My boyfriends brother is uncomfortable around me?
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