Hi OP.
I feel your pain, but wanted to put a different perspective on it. Long story short. I met my dh when we both had one child each, and for several years both dc would go away eow. We had regular alone time and it was heavenly, then we got married and had two more children in rapid succession, and -poof- no more time alone. Ever. I found this really really tough. He didn't so much, I think.
Just wanted to say that them being dsd's is not the issue. I felt the same about my own dc, despite loving them deeply. And I do understand about feelings toward a dsc, as I have one of those too.
Your issue is with your DP not making any allowance or room for you in his family and expecting you to make all the adjustments. I don't think running up and downstairs after bedtime is reasonable and normal with children their age, he is treating them like babies and it really needs to stop now. The intercom needs to go.
I suggest explaining that you are finding it harder than you imagined and ask if he can see anyway that he could help you adjust. Ask if if he thinks it's normal to be at the beck and call of his DD's all evening, I suspect this has been borne out of some sense of guilt or sorrow at their not having their dm around, he probably isn't particularly enjoying it either but that it has become such a routine since they were babies that he no longer questions it. The whole dynamic of their relationship as a 3 will have to change if you are to join it and make it a 4. To think you can slide in but remain on the periphery for ever is not fair on you, and will not help the girls to accept you as a part of the family now. You really need to understand from him that he will be able to make changes to incorporate you into the family, which will mean you all having to make some changes and some sacrifices. These can be gradual, and one step at a time, but if you can together make a plan and you can see the small efforts and changes that will include and consider you (and him together as partners) which will in turn have a beneficial effect on the dsd', I think you will be able to manage the more difficult moments.
The bedtimes seem to be at the thorny end of the stick right now, and I do think these may be a starting point but there will be lots of challenges ahead, such as how will you be viewed in future - as a mum, or always as daddy's girlfriend? As there is no mum on the scene the former would be a natural enough position to take up, but with it comes active parenting, setting boundaries, and carrying out sanctions or punishments, doing the school run, taking them shopping, advice on bf's, advice on periods, contraception, managing hormonal stroppy teenagers - all ahead of you and cannot be done without conflict. Read any of the teenager message boards. You need to really think about what you are taking on, and accept it will not be a bed of roses.
You need to talk, really talk, to him if you hope to develop your relationship and stay together. Many dp's of the same biological dc don't manage to successfully co-parent, so them being not yours makes it even more important that you talk to each other, are honest about your feelings, and accept that you both need to accommodate each other, and so do the dsd's. My dh and I have precious little 'alone' time together but aim to have at least a couple of kid free evenings a week to chat, watch tv, snuggle (by putting them to bed/asking teenagers to be in their bedrooms).
I hope I haven't put you off
I have all flavours - own kids, step kid, sons, daughters, teenagers, little ones and I love them all dearly, but individually, and I wouldn't be without them. I love my dsd differently to my own dd and I love my two ds's differently from each other and from their sisters. And it grew, my love for them. It didn't happen like a thunderbolt overnight. It took time, and so I think it should.
Good luck OP, I hope you fathom it out (and if you do come back and tell us all, we will be grateful for your insights!!). Xx