Relationships and Bonus Kids

I am really struggling. The guy I am marrying was with his ex for 11 years, married for 8 and she had 2 kids from a previous relationship. Those kids see him as a father figure because he was around for so long. They are 16 and 18 years old, both female. I have tried so many times to build a relationship but its not working due to their mom being someone that doesn’t approve of mine and his relationship. The girls will constantly belittle me, use him for their own benefit and ever since him and their mom divorced, they have become VERY touchy feely with him, other people also notice how inappropriate it is. He says that its nothing due to the fact they are his kids. I love this man more than anyone I have ever loved but I feel like his girls are trying to tear us apart and I dont know what to do. It’s like I’m not allowed to hardly even talk to him if they are around. He doesn’t notice it and I haven’t said anything to him because we both agreed in the beginning that our kids would always come first. I feel hurt and lost anytime they come around.