I've been with my DH now for just over 2.5 years. Things moved pretty quickly for us but we'd known each other 20 years before we started dating. We actually dated in 1st year in high school for like 3 weeks (super long-term when you're 12).
He has 3 kids and the custody split has always been 50/50. I met them when they were 3, 13 and 14. I'm super close with my now 5yo DSD. And possibly controversially, I am like a 2nd mum to her. But she needs that. Her mum isn't great, doesn't do much with her so I feel like I have to pick up her slack. My DH is a wonderful dad, very present and very hands on with the kids. The exact opposite of a Disney dad. He was always the one to lay boundaries and implement discipline. I do a lot of the "girl stuff" I feel DSD misses out on with her mum like going to the hairdressers, going clothes shopping, baking cakes. I even threw a mini hen party for her and her friends so she felt part of the celebrations and build up to the wedding.
DSD copies me a lot. She has a vitech laptop and will sit next to me when I'm working from home and type away on her laptop. I recently got my hair cut into a bob, guess who else now wants their hair cut short. My DH says there's worse people to copy.
DH and his ex split because she assaulted him for the 4th time that year and DH had already earned her he'd leave if she hit him again. She bit his face and headbutted him Infront of their then 2yo. She also had several affairs and generally just isn't a nice person. She's very selfish, rarely puts the kids first and will use them as weapons to try and hurt DH. She currently has a BF who is married with 3 kids. They had an affair before he eventually moved in about 2 months ago after his wife found out.
The ex makes life difficult. We have boundaries in place to minimise her impact but simple requests are always an issue and she'll throw curveballs just to fuck shit up now and again. She stopped us seeing DSD for 2 months over an argument about a dog that she has since rehomed.
I get on ok with the 2 boys but I wouldn't say we're close. They were teens when we met so I've never been a parent to them though they both do refer to me as their stepmum. They're typical teenage boys, sleep all day, glued to their computers or out with friends.
I think the key to a happy step-family is having a partner who has your back 100%. Who respects your opinions and makes time for you to be a couple. Who can prioritise the relationship as well as his kids.
I do worry that my relationship with DSD will change as she gets older, especially since her mum badmouths us, however DSD even at 5 is already starting to see through her mum's BS. I guess we'll just need to cross that bridge if we ever come to it.