My boyfriend's little sister is jealous...?

My boyfriend's sister is 7, and she always gets jealous of me when me and my boyfriend try and do something alone, she'll cry, sulk, and get angry. She also draws pictures of her crying and me being mean to her. When I am not with my boyfriend and just with her, she likes me a lot, and she always wants to play with me when I come over. But when I try and hang out with my boyfriend, she turns into a little devil.

My Boyfriend is really nice, but anytime she sulks or cries because we aren't paying attention to her he rushes over to her and gives her attention and plays with her. This means we get almost no free time. Its getting to the point were he'll even bring her over to my house when he comes to visit for the evening! I have no idea what to do...
My boyfriend's little sister is jealous...?
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