I could have written your post.
Except that things did worsen when her sister was born, and have improved some since.
I have come to the conclusion that this must be a developmental thing. Lilah was also always super easy at bedtime until about 2 months after she moved to her big girl bed. Then she started with the million excuses routine and playing and talking into the late hours (we're taling she wasn't asleep before 11 pm most nights). I shortened her nap, which helped, but didn't solve the problem. Right after her sister was born the tantrums started. They are much better now, but still happen.
Lilah also is a complete monkey right up to bedtime.
Honestly, I really don't have too much help to offer. I have started making sure her bedtime routine is very quiet and calm, with dim lights, singing in the dark right before tuck-in, etc. I don't think anything has helped to much. The only thing that really does make a difference is that when DH has to work late and I do the bedtime routine myself, the fighting is greatly reduced and she goes to sleep much quicker. I don't know why, and it certainly isn't worth it to have to do both girls' routines myself, but there is a definite difference. Also anticipating some of the excuses, like putting a sippy of water in the room, and making potty visits ahead of time help a teeny bit. But mostly she just wants me or DH to come in. And then she thinks it's time to play once we're there.
A couple things I plan to try:
--"get out of bed" passes. I'll allot her two. After she's used them up, their gone. Even if she needs to pee yet again.
-- trading in the passes she doesn't use for small treats the next day
I'm betting, though, that we probably just have to ride this out.