Hi Pevie
It sounds like your dd has a sleep association with you/being rocked, and the only real way you're going to get her to sleep for longer is to break that association. We all have sleep cycles of 35-45 minutes, and when she stirs after that time, she's expecting the same surroundings/sensation as when she went to sleep, and when she doesn't get it she wakes up.
I wouldn't try CC - she is too young anyway, and will just feel abandoned. If you can find the time, then have a look through the Baby Whisperer book or website, and see if what Tracy Hogg suggests would work for you. Briefly you train her to go to sleep on her own, in her cot/moses basket. To do this, you ensure she is laid down awake (but drowsy). She will cry as she wants to be held, but try putting her on her side and rubbing her back and shushing her to calm her down. If she cries a lot (rather than in a tired way) then pick her up, soothe her and as soon as she stops put her back down, and continue with the back rubbing and shushing.
Expect to spend some time getting her to go to sleep - longer than if you rock or cuddle her, but eventually she will go to sleep on her own or with minimal assistance, and should then sleep for longer too and better at night. You might need some help with dd1 while you're doing this.
All children are different, but I followed this method, and ds was sleeping through from 11pm to 7am at 12 weeks, and has now been sleeping through from 7.30pm to 6.30am/7am for a few months (is now 10months). Get the odd night waking of course. I didn't use this method with dd1 or dd2, and I used to have to cuddle or lie down with them to get them to sleep. In desperation I used CC at about 18mths, it worked but I hated it, and think this method is easier and as effective.
Good luck...