Dating someone adult children!!?

Has anyone dated or married to someone with adult children? How is your relationship with them, communication etc.?? I need advice, me and my fiance have been dating for almost 5 years and are getting married in July he has 3 adult children from his first marriage 2 are older than me by only a couple years and one is younger. I've felt disrespected by them on a few occasions, they don't talk to me when my fiance and I visit them I usually have to spark up conversation like how's everything going etc. They just give me one word answers. They don't live in the same city as us so we only see them once a year. We have a baby, 4 months old and they hadn't met her until this week his oldest son was in town for business he stayed at our house but my fiance was out of town so I was left with the hosting. He literally looked at my baby and said "she looks just like you she looks nothing like my dad" that's it for the 2 days he was there he didn't hold her talk to her nothing. I found that weird. Neither kid has ever said anything mean to my face but ALOT of subliminal things. His oldest also had a baby 6 months before I had my daughter his kids flooded fb with posts when she was born my niece is here posting pics of her congratulating etc. Mind you they weren't there for her birth either but when I had my daughter no one reached out to me no text no congratulations no nothing not to my fiance either and that's their sister. Maybe I'm being sensitive or taking all of this too personal maybe I should just not even worry about it. But I feel like with us getting married it's becoming harder to avoid the awkwardness of it all. For whatever reason they don't like me. Probably bc of my age! But I just wonder am I the only one should I try and reach out more or just say screw it!? I've brought it to his attention before and he just says they do like you, they are happy that I'm happy he says I'm being too sensitive. Should I tell him that his son didn't even interact with our daughter? I really have no one else to ask about this any advice is helpful BTW I'm 26 and he's almost 47a