[Nightblood.]

First, I don't think you can compare the way he's going to love his girlfriend or future girlfriends and the way he loves you. It's a completely different kind of love, so while he'll love them differently I wouldn't say he'll love them more.

It sounds like he may be doing this because he knows his girlfriend makes fun of her sister and that she sees it as normal, perhaps he's afraid if he doesn't make fun of you then this girl will see him as weird. I think you should try talking to just him about it, maybe bring up that he only started saying these things recently. If that doesn't work, I'd just ignore it.

Finally, I think your expectations for friendship with his girlfriend is too high. In my experience, when you want to be best friends with someone (especially someone you don't actually know) it usually doesn't work out the way you think/plan. It's also very likely this isn't going to be the girl he marries.