long lost child

this will be a little bit of a long post so please bear with me.

my fiance and I have been together 4.5 years. When we first met he told me he didn't have any kids. fast forward a few years and he tells me he may have a kid but not sure. he shows me a picture of her and there is no denying that she is his child. now fast forward to yesterday and he liked one of her posts on fb and she responds "how do I know you?" so he is supposed to talk to her some time today and explain the whole situation as to why he hasn't been there in her life this whole time. I'm very supportive of all this with him and even excited and nervous for him. but o can't help feeling that once she comes into our lives, I do have kids from a previous, that he will forget about us or start treating my kids differently. am I justified in feeling this way? I know it's a little selfish of me but can't help feeling this way. what father wouldn't want to know a child they havent seen grow or get to know? I don't want to bring it up him cause I'm just letting it play out for right now. I don't want to steal his joy. some insight would be wonderful. thanx