Reign: Tell him blatantly (but in a civil way). You have the right to talk to him about the situation because it involves the you and the whole family. Maybe it's just in the beginning that you're uncomfortable. Be nice when the girl is around, but don't let your guard down. Cause you cannot really trust her that easily. Tell him that you need time to get used to it. This of it this way, she's probably more uncomfortable than you are.
Anna: Cook him some breakfast. Brew some coffee, and sit in front of him. There you trapped him! Now talk to him. Just let it all out as gently as you can because he still controls your allowance. Although examine the situation first. Why don't you like her? Have you ever given her a chance for you to like her?
"I don't like my friend's significant other, how do I tell her without offending her? It's 'cause he can be a bit rude and douchebaggy sometimes..."
Reign: There's no other way to tell her but to say it straight. Like if I were you, I'd seriously talk to her and tell her something like "oh, your boyfriend is kinda rude to me and I don't think that's right." Would you want your friend to date someone who is only nice to her? If she gets mad, then fine. We can't do anything about that, at least you've said what you felt. Remember: VERITAS.
Anna: Well it all depends on how much of a douchebag he is. Maybe he's only a douchebag to you. Kidding. But really, maybe he's really nice to her and he shows a completely different side when they're alone. However, if he is abusive to your friend (eg. asks for money, cheating behind her back, sex all the time) then that is you better step into the problem and make this your business. Tell her as lightly as possible, and if she doesn't take this seriously, you can get fucking mad.
"What should I do about my insecurities? I really want to become a better person and to be somebody that other people can love but I feel like my selfishness and insecurities get in the way. Like I have a hard time when my best friend has other friends or ditches me for other people because I feel like one day she'll realize that I'm not great at all and I'm just a mess then she'll leave me. What should I do? How do I fix this? I hate giving my friends a hard time."
Reign: You have to establish why you feel that way about yourself; maybe you have some things to improve on, maybe you've been feeling out of place lately or maybe you're just being paranoid. You have to learn how to accept and understand yourself, that way people will eventually understand you too. Appreciate the kind of person you are. If your best friend really is your best friend, she wouldn't leave you just because of how you think about yourself. She would be there to help you get through what you're dealing with. I know it feels kind of "wrong" to burden your friends with your problems but they're your friends, they will listen to you.
Anna: Who told you you were a mess? Who said you aren't that great? I mean, I see my friends and their flaws but I wouldn't make a big deal put of it. Listen, for your friend ditching you (well not exactly ditching) don't think just because she has other friends means she doesn't like you. She's just friendly. So let her be. Let her have space. You though should start expandig your group of friends. It may be hard but it is totally worth it when you see that people do like you.