If a step-dad can can chime in...
I find it difficult sometimes because the girl's dad only parents on request. He does do parent's evening, events etc as long as he's reminded the day before but he wouldn't ever notice if he wasn't informed. My girlfriend simply strives to keep him involved as much as possible, for the girl's benefit and to be honest, for his too.
Meanwhile, the girls live with us 12 days out of 14 and I try to balance basically fathering them full time with tying to work out where this line is that I shouldn't cross. They're only 3 and 7 and basically the focus of my life. I love my girlfriend so much and couldn't contemplate losing her but the thought of ever losing those two girls cuts so much deeper.
And then there's days like when the 3yr old started kindergarten and their dad calls in the evening. My gf starts telling about the little one's first day and he says he's not really interested, he just wanted to chat with my gf. I'd taken the next day off work and arranged to work from home the day after because we agreed with the kindergarten that she should be eased in with short days to begin with, and their dad couldn't be arsed to hear a couple of sentences about how she had done?
He's still in love with her (it was a very dependent relationship in which she basically mothered him, which I think is why it's hard for him to move on.) I don't blame him obviously. I never have even the first moment's worry about my gf's feelings, so that's not an issue, but yes, it's difficult that they are by necessity close still to co-parent the girls.
He's actually doing a better job of showing an interest and making the most of his weekends, to be fair. And to his credit he doesn't just totally Disney Dad them either - they might get more candy but he doesn't spoil them in an over the top way and they undeniably come back happy. And we actually get on pretty well with eachother. He hates me in the sense that I'm with his ex, but actually has no issues with me as a person.
It's just difficult. The bond him and my gf will always share, the knowledge that he will always (and rightly so) mean more to the girls than I do, the slightly infuriating knowledge that he isn't quite shit enough for me to really feel hard done by over it.
Hmm, sorry, I ranted a bit. Life is actually really good and I don't regret deciding to invest everything in this family for a second but maybe I bottle one or two frustrations.