S.T.
Sounds like she needs reassurance that you're there, will be there and will always love her - even though you've brought this little interloper in to the house. (as I've heard illustrated - bringing a new baby home is like your husband bringing home new wife and saying "I'll still love you the same as before"). Ask her how she'd like to help with the baby. Follow her lead. She may also be going through a developemental surge - either physcially, emoltionally, mentally. Just when we figure our kids out they grow & change. It's not fair!
As bed time approaches set up expectations for the night. Tell her you know that she can sleep on her own and you expect that she'll do that tonight - and how proud you'll be when she does. Marke sure she's getting a lot of outside physical activity that will tire her out - and have a calm environment around bedtime. Set up a goal - after 5 nights through you'll get her a treat (inexpensive toy she really wants, trip to her favoirte playground, etc.) and make a little chart with stickers for that 5 day period. - so she can see her progress each morning. Have her help you make the chart, select the goal-prize, pick out the stickers, stick the stickers each morning. Remind her that she's able to make these decisions and take control becuase she's maturing from a little girl to a bigger girl. Remind her that it's sometimes a little difficult to grow up and leave one stage behind - but you look forward to her growing in to the next stage. Then move it out to 10 days, etc. If she gets up in the night go in to her room put her in bed and tell you you'll sit on the floor by her bed until she falls asleep - but no sticker that morning.
Positive reinforcement all the time - ignore the unfavorable outcomes as much as possible. Set up expectations in advance. An 80 year old sunday school teacher taught me this when my kids were toddlers - my kids are 11 & 14 and I still use it -adjusted for age of course!
Best of luck!