Well, I suppose from their perspective you, your DH and your existing kids are not 'new', kwim? So maybe it's not that they are dismissing you or the importance of your involvement, but that it goes without saying that they'll be seeing you. If someone brings flowers, they're for you, not for the baby.

Especially in the case of parents and family (cos it's usually family who say such things LOL), they've been seeing you for your whole life, so they're pretty used to it by now. In the same way that breathing is one of the most important things we do, but we don't really think or talk about it much, we just do it. So in some ways seeming blase about it might actually be a bit of a compliment about how important you are in their lives; so important that it just happens without having to talk about it much.

My mum always made me laugh when DS was tiny. She lived interstate and whenever she'd visit she'd be so excited to see him that she'd rush in and start hanging out with him without a thought for anyone else. After about five minutes I'd say 'Hi Mum', and she'd look up, realise that the rest of the universe still existed and get all sheepish. But it was hard getting mad at her about it because her excitement was so infectious.