How many hours/what hours would you like her to sleep through? Like 7p-7a? Or 11p-7a? If you can choose some hours that you'd like her to sleep through (and it'd be good to take into account her natural sleep/wake rhythms), then you can be consistent about enforcing them in a gentle, gradual way.
Consider too that she might have a physical reason to wake - perhaps to pee, or she could be thirsty/hungry, etc. - do you, as an adult, not drink or get hungry in the time you want her to sleep? I only ask this because I think sometimes that we expect more of our kids than we'd be willing to agree to as adults.
Her waking could be due to one or a combo of:
* adjusting to having a new baby in the house
* thirsty or hungry (growth spurts can be vicious!)
* full bladder is causing her to wake up (if you take her to the potty, let a trickle of water run in the sink, or make a "sssss" sound, she may just pee for you and go right back to sleep)
* some other physical discomfort (teething - 2nd yr molars maybe?, or oncoming illness)
* emotional need - needing to know that her mom is close by, still around given that there's a new baby vying for attention (plus, 18 months old is still very, very young - still very much a baby!)
I suggest trying a variety of things to see what works - here are just 2 ideas:
* plan to wake just before the time she typically does (you mentioned that she sleeps for "about 5 hours" before waking for you?) and sleep walk her to the bathroom, support her on the potty and give her an opportunity to pee
* put a small sleeping bag in your room and allow her to sleep in your room for those last few hours in the morning (so she's not in bed with you, but she's secure knowing you're right nearby, in her line of vision)
I can only imagine how busy & tired you must be with a young infant and a young toddler! Mine are just under 3 yrs apart (my youngest is now 7 mos old) and we are still absolutely exhausted and trying to get a handle on parenting 2 kids at once - the nighttime sleep is the least of it for us (at least they're both asleep simultaneously for stretches of time!). A tip from a friend of mine who had her babies 15 months apart - she read a book on raising twins called "Mothering Multiples" by Karen Gromada:
http://www.amazon.com/Mothering-Multiples-Breastfeeding-C...
Even though it's targeted towards moms of multiples, my friend said she found the book to be really helpful in juggling two kids under 2 yrs old. Hope this helps, and that you can find more sleep together as a family!