Do you believe that you are an important part of their life, or are you - as I saw described on another thread yesterday - just background noise which they tolerate/accept with little attention?
I tried hard to find a role in my DHs DCs life, and I believed the people who told me they'd spoken highly/affectionately/fondly of me. We operated as a family - holidays, days out, meals together etc and DHs DD even worked for me for a while, but I now realise that I was never more than a passing acquaintance to them - I haven't made a lasting impression on their lives and after a few years of no contact, they probably won't even remember my name, I'll just be that woman their Dad lived with (and that their Mum didn't like)
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I'm wondering if I'd have behaved differently towards them if I'd known - and was it a good thing that I didn't know?