M.T.
Hi, L.. Congratulations on your pregnancy! I have two children, and one of the best parts of motherhood was seeing the sibling relationship develop and flourish between my children.
My daughter was 4 when my son was born, and there was absolutely no jealousy, sibling rivalry, regression, acting out or anything like that, she made a totally smooth transition.
It's great to get her involved in planning for the baby and letting her know that the baby is for the whole family. But it's important to remember that at 3 1/2, she is still a really little girl. I think sometimes parents and other family members expect/demand that the older sibling become more mature, more independent simply because there is a new baby in the family, and that's not the case. Her needs are still the same 3 1/2 year old needs after the baby comes as before. A lot of parents seem to make a big deal of the older child being the "big sister" or "big brother" but remember that the older child has their own identity, and shouldn't only be recognized as the baby's older sister or brother.
There were times after I had my 2nd baby when I would actually make a point of letting my daughter know that sometimes she still came first - like when the baby would fuss and I'd say "I'll come and pick you up as soon as I finish making K's lunch." This was for my daughter's benefit of course, the baby had no clue what I was saying!
Good luck for a smooth sibling transition!