I feel your pain Queazy, I really do. Reading between the lines (and probably filling in from my own experience) I imagine that you love both your babies but are finding it bloody hard work to like your older DD right now. I just want to say that it's ok to feel like that, and that it passes. Don't get upset at yourself for having such feelings - it makes the whole situation worse.
As a personal digression (feel free to skip this): DS1 has, to me, always been the contrary one. If you plan a wonderful day that he will, in principle, love, he'll throw a strop at the beginning of it and be a right bastard all through the day. I'll be there with my smile fixed on, trilling 'We're going to have a LOVELY time, darling!' while all the time in my head I'm screaming 'Why do you fucking do this EVERY TIME, I'm hardly going to the fucking park for my own amusement AM I?! It's all for you, you ungrateful OIK!' DS2 will be pootling in the background and being agreeable because he's easy that way, bless him, but DS1 can be fucking hard work and seems to delight in thwarting my plans and making me sigh and go "Oh, fuck it. I don't care any more. Sit in the corner and stare at a screen and eat fucking Pom Bears all day, I could not give one shit." He will then crawl all over me and whine to do the thing that he refused to do in the bloody first place 
So, yes. Ahem. He is now 4.8 and light-years better than he used to be, we get along quite well most of the time (see above for thankfully now-rare episodes
), I love him, he loves me, and he and his brother are so close it's untrue. So from my limited frame of reference, it all works out. It's just hard work getting there 
In practical terms, we put a Moses basket inside a travel cot so I could put DS2 down to sleep without DS1 assaulting him. Worth a try, maybe? You could attach a blanket to the top of it with cable ties or clothes pegs so DD1 can't drop toys on her sister.