Clingy Sister!

My boyfriend and I have been dating for a little over a year now, it's pretty serious (we've talked about marriage) and I'm very happy about how our relationship is looking, except for one problem. His 13 year old sister is ridiculously clingy, but only when we're together; she even threw a temper tantrum once when we wouldn't let her go on a date with us (we're talking falling on the floor and whining like a 2-year-old)! I thought if I bonded with her, she would understand that I wasn't just trying to take her brother, but that just made it worse. Now she thinks whenever I come back from college, I'm there to hang out with both of them, so when we're together without her, she blows up our phones until we get back. Any ideas? What do I do?? :(

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