My name is A., I am a middle child and mother of six, yours, mine and ours... Three girls and three boys. My baby is now 17. Our oldest daughter has two children, girl 7 and boy 3. My sister has two, the oldest a girl 25 and a boy 24. My brother has three boys 11,9 & 8. My girlfriend has three, the oldest is a boy 11 from a previous relationship and then two girls 4&2, from this marriage.
I gave you all that information to say that I rarely see Any jelousy that is ongoing. Now and then a situation might come up but it passes. I believe, as you said, you felt jealous because even as a child you were aware of the difference between the two of you.
I learned when mine were little that the older they get the less one on one attention they need. They learn to enjoy themselves and their own intrest. Just make sure you do give her some undivided attention. Make sure you take time to realy listen to her too. (We can get distracted and not realy be listening, they know it too)
Don't stress over something that is not there. Your daughter is old enough that you can let her know that a baby needs more attention. You might want to make special time for you and her to do things alone. Movie, out to lunch, a trip to the library to pick out a book that you will read together. It will make you both feel better. The most imptortant thing is that you don't put yourself on a guilt trip. She will pick up on it and play you when it's convienent for her. (We're all human, even 6 year olds)Children are very smart.
I hope something here has helped.