Agree with lynette! Your DD sounds overtired and needs more sleep, most of it at bedtime!! I totally didn't realise that you HAVE to make sure babies get naps during the day until my public health nurse mentioned it... and said DS looked exhausted. 
Babies of this age can't stay awake for more than 90 mins to 2 hours at a time. If they do it's because they've caught a second wind and are actually wired/overtired.
Since 5 weeks this is what I've been doing with DS (now 10 weeks) with reasonable success. I aim for him to have 2 short naps and 1 long nap during the day - totaling 4-5 hours as Lynette says. One short nap morning tea time, longer nap over lunch and short nap in the late afternoon. For the late afternoon nap - if he wakes from his big nap at around 2pmish I try make sure he gets an hour-long nap in the late afternoon. If he wakes at 3pmish it's ok for the late nap to be only half an hour. He then gets settled in bed/in my arms for his nighttime sleep no later than 2 hours from waking from his late afternoon nap (eg about 6.30ish). It can take up to an hour to get him settled to sleep properly for the night after that (rocking him asleep in my/DH arms with pacifier after bath and bottle seems to work best right now) - but after that he'll sleep through with sometimes just 1 feed in the middle of the night and maybe another at 5am.
It's really hard to try and get him to nap late afternoon but I simply have to do otherwise he goes bananas at bedtime and can scream for anywhere up to 4 hours before conking out with exhaustion (and exhausting and upsetting me in the process). Usually for the late nap if he's not going to doze in his bouncer in the living room I just have to get out the house and take him in the pram with blackout blind, rain or shine.
As for feeds and naps, if DS needs a sleep but isn't really hungry, I would still try give him a small top up as long as it's been more than 2 hours since his last feed. Not sure about your DD but it's impossible for me to settle DS for a long nap if he's within an hour or so of his next feed. DS needs sleep more than I need to worry about wasting most of a bottle. Usually now we're into the swing of things his naps come after his usual feeds, though.
You need to watch for the signals she is tired (I totally didn't know these until my health nurse mentioned it!!):
- bringing hands together in a fist to chest
- yawning
- staring into space
- getting grizzly
As soon as you see these signs, get her down for a nap straight away - BEFORE she really loses it and she's overtired. It's hard at first but you really have to perservere with it - darkened room, swaddle, pram ride, car ride, whatever. I start watching DS carefully when he's been awake for 1 hour+.
Also if you can avoid having visitors or having baby interact with anyone other than the usual people in the late afternoon (4pm or later) - I find it really winds DS up too much and makes him noticeably hard to settle. MIL gets all huffy about this but you know what, she's not the one that has to settle him when he's desperately overstimulated, overtired and screaming!!
So in summary: flexible routine is your friend and MAKE SURE BABY NAPS!!
Remember babies don't know how to fall asleep when they're tired, we have to teach them!