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Honestly, there's no real way to know if they will ever mature. Some people live their whole lives without ever maturing. You've only got a couple of choices, one is to ignore them and don't ever let them see that they have upset you. As time progresses and things remain the same between you and your boyfriend, they will see that you are not affected by them and that you're not going anywhere and eventually they will have nothing to fuel their little games. What is people talking about you consist of but jealousy and fear, right? The other option is to approach him about it and tell him that you don't appreciate it and if he doesn't speak to them about it, then you'll consider leaving. The problem with ultimatums is that sometimes they choose to leave, so you would have to be prepared for that.
I'm sure you may have already tried to befriend them. If not sometimes the way you can stop things like this is to 'kill them with kindness'. Invite them over for dinner (in front of their father, so it's hard to say no), every time you see one of them, compliment something about them. Tell the oldest the things you like about her (i know it may be hard, lol) perhaps you like her free spirit, or outspoken attitude or her taste in clothes. Ask her if you can talk to her about a problem you're having. Ask her to go shopping with you, or to help you pick something out (perhaps a gift for her father). Sure all of this puts you in a position to get rejected, but take it with a smile and ignore it. After a while, they will see that nothing they do can affect you, and the nonsense may stop.
I hope this helps. Let me know if you want to talk more