I am 12, 11 and 9 years younger than my older siblings. I love my brothers and I am glad to have them but at 37, I am still "the baby".
They had a different childhood than I had. They grew up with built-in playmates. Our parents were older and more established when I came along. I went on cool vacations with just my mom and dad as a preeteen, teen. My brothers went and spent a week at Grandma's house, who was not healthy enough for extended visits when I was around.
We never really fought about anything. I think that we are close, and there were some aspects of being so much younger that were great. The best thing is that I got to really enjoy being an aunt! I have nephew and nieces for years before I became a parent myself. I feel like I really got to spend time with them that I wouldn't have been able to do once I had my own children.
The other thing is that my children are the youngest grandchildren. My brother has a daughter the same age as my oldest but my youngest is the youngest, on both sides of the family.
We had our children 2 and a half years apart. I love that they are close in age.
I think that you will just have to let your daughter adjust to the idea of a sibling when it is actually going to happen. I wouldn't bring it up now, because that is an awfully long time to stew over that for a little girl. I would let her know when there is a baby on the way. If she asks you if you are ever going to have another baby, then I would honestly answer her, but I just think, until there is actually something for her to fret over, then I would leave it alone.