I had to address this one. I am also new here and just aired out my own situation to get thoughts and feedback and I really liked my responses. I first and foremost can empathize with you completely although my situation is slightly different. I remember in the beginning feeling very jealous of the kids and of the way I felt like I would always take a back seat to them in my SO's heart. It's very hard. For me the first year of the relationship it was just us...I was aware of the kids but they weren't, like, full time in the picture yet. I am not sure if I would have entered into this thing if they had been there from day one honestly but I fell in love so what came next I decided to stick around for. Girl...take your time deciding is all I can say and even if you decide you can take this on...be aware that nobody can tell you how hard it's going to be or exactly what is coming. Play out all scenarios in your head. Suppose something happened down the road and your SO had made the decision to move one or both of them in with you two. Could you live with it and could you be happy doing it? If they even moved closer and you were suddenly seeing a lot more of them how would that sit with you? Can you share the most special times of year with them knowing it will likely mean you feel like a third wheel? And I have personal experience with watching kids take advantage of my partner and GIRL!!! It is so hard to watch and you want to protect your SO from these situations but the SO doesn't even see it as taking advantage because these are their children and that is SOOOO HARD to deal with!
It's possible that all the soul searching in the world will still not prepare you for how challenging this is but still take the time to do it. It sounds like you are already to some degree of course but I would just recommend that you play out every possible scenario under the sun no matter how unlikely it seems and really think through how you would feel and how you might deal with it. Perhaps even talk it over with your SO. From your post it sounds like your SO is great and wants to help you not to feel left out but there are just times they cannot save us from ourselves. I am also curious to know if you want kids of your own down the road? If so do the two of you seem compatible and agreeable on how you would raise children?(had struggles in this department myself and my God is it difficult)
Anyway I hope I have contributed in some way here girl. I don't think you are selfish at all for what it's worth. I believe we all need to feel as though we are somebody's #1 and sharing a partner with their kids can make even the most secure person feel as though they are taking a backseat. It isn't for everyone!!!

I think for some people, the feeling of being alienated never goes away.

If you're lucky, your stepdaughters will accept you and maybe in time they may let you into their circle with their dad.

For most of us, however, the kids are standoffish and refuse to accept us. My stepkids absolutely loathe me and will never include me in their circle. I've learnt to accept that and keep my own separate circle with my husband. But every time he interacts with them, I feel left out - just like you. If I'm really honest with myself, I think I get jealous even. }:)

If you have a good partner, he will feel your pain and try to minimise it. The problem is when your man leaves you to sort out your feelings on your own and "sides" with his kids. You need to be honest with yourself and figure out which type of partner your man is - then make the decision to stay or leave.

Good luck either way. Just know that what you are feeling is TOTALLY NATURAL and not selfish at all. Don't beat yourself up over it

I suggest you seek counseling, you have some underlying issues that needs dealing with.... could be childhood related or first marriage related.

DO not get married again until these feelings are gone, or dealt with. Keep your relationship as is... yes you are ready for the next level in your hart but not in your mind Hon, deal with it first...

You are lucky to have skids in a different state, but you and SO need to be on the same page, he needs to understand that feeling guilty about a divorce does not need bags and bags of money to be spend on children,

I would not marry this man until the last kid launched... you are not married yet, what will you do if SD15 suddenly decides she does not like mum's rules and wants to live with her Dad, permanently, are you able to handle it? What what are you going to do if the oldest SD decides she's coming to live with you and Dad.... see these are major problems.... I hope you and SO talked about it and that it's made clear from your side, that when you started dating you knew he had children not living with him and that's what you signed up for..... you did not sign up for children suddenly moving in.... see why I say keep marriage off the table for a while longer...

Also sort out the holiday situation, I'm not there to observe and I'm not in your shoes, when the daughters are there, how does SO react, ignores you and treats you like a second class citizen, allowing his daughters to disrespect you and do what ever they want, SO stop being attentive towards you, you are no longer his first priority.... see all these things you have to sort out before you even think about marriage or engagements....