Fiancé’s close family

So my fiancé is amazing and he has a wonderful loving relationship with his sisters and brother. They all get along really well and they text everyday all throughout the day and have exclusive sibling dates. It’s really beautiful honestly and I’m so glad they have each other. Me on the other hand, I have tried to be closer with my two brothers and we do love each other but now that we are all grown up and out of the house we are not so close anymore, I have reached out to my brothers and tried to change that, I feel like life only gets harder as we get older and the older I get the closer I want to be with them. They have not really reciprocated to my efforts, they kinda just want to have a talk every now and then relationship and catch up on the holidays.

Sometimes when I am around my fiancé and his siblings it makes me sad because they are so close and it reminds me of my own family and I wish mine was like that. It’s not like a wish them badly because I’m jealous kinda thing, I’m honestly so glad they are close, I can’t help being sad and feeling left out because of my own situation though. I don’t want to tell my fiancé this, but sometimes I’m worried if he notices I’m sad and so I just keep my distance sometimes. I don’t know how to stop these feelings I have and I feel ridiculous for having them in the first place and I feel lonely. Can anyone offer any insight?