This is VERY normal!! Many kids do this. It's just that their brains are going so fast and their little mouths and tongues just can't keep up! My daughter did this around 2 yrs old and she was/is very verbally advanced. She stopped by 3 and she's now a 4 1/2 yr old reading at 2nd grade level, very articulate with a great vocabulary.
Don't worry about it. Just let her take her time and get the words out. It can be very frustrating for the child - they have SO much to say and just can't get it out fast enough! Even if a child is a true stutterer, my doctor said it wouldn't be evaluated or diagnosed at such a young age. My son, now 3, does it now. He's not at all as verbal as his sister and he seems to do it worse sometimes. I'm told if it's still strong at about age 5, that's when it can be assessed. Perhaps having her new brother around has contributed to her frustration and makes her that much more determined to assert herself, but I wouldn't worry about it being a "cause."
PS - if you really want to have her checked, your ped. can give you an Early Intervention recommendation. They will come out to your house and do an assessment. At this age, it would not come from the school district. (Our son was evaluated by IE when he first came home... see below)
PSS - our daughter is bio, our son was adopted from Russia. Both are such blessings... maybe your husband will open his mind in the future. We are so lucky to have them both. Not that there's anything wrong with having an "only!" Just that people's minds can change. :)