M.,
I had the same concerns with my daughter, who is now 5-1/2 yrs. old, for the same reasons. She is my only biological child and her brother and sister are much older. Although she went to a sitter, most of the full time children were older until she was 3, when the sitter (luckily) got a few more children, all in her age group. That is when I noticed the change in her social skills with other kids.
My suggestion is, if possible, find a private sitter for her, preferably one with other children close to her age, that you can send her to either full time or a few days a week. I know a sitter may sound scary, but any good sitter will have years of experience and references that you can contact. Most times a private sitter is less expensive than a full time Day Care so that should help as well.
If a private sitter is not an option then once she gets a little older, look for programs at your local library (ours has a Story time) or with your city's Rec Department for things that you can get her involved in. Little Gym also offers lots of classes for different things, although they are a little pricy.
I hope my suggestions are of some help.
Good luck!
S.