Hi M.,
I'm a preschool teacher and a mom, and my little boy was pretty similar until recently. During the summer, he wanted *nothing* to do with most other kids his age, besides the older girl (she's in kindergarten) who lives across the street, and I believe that started only because I traded care with her father. He does very much, still, prefer older kids and will still get into some parallel play with his peers at preschool. The teachers there have been great about finding more outgoing older children to just sort of grab him by the hand and bring him into what they're doing. Otherwise, he's quite the hang-back-and-observe type.
In your situation, the familiarity of your daughter's cousin may make things feel more predictable for her. I don't know your daughter, and it may likely be more of a comfort-based choice not to engage directly with the other kids than it is about language; I've taken care of some very articulate children who were just more comfortable with adults/older kids/familiar kids than the group at large dynamic. Sometimes, children with Aspergers have a harder time picking up the social cues that come easily to other children. That said, I have also seen this corrected, too. However, if your pediatrician isn't worried, I'd take that off my own list of worries until proven otherwise.
And, (I'm going to add an AND here, but it's important) it may be that your daughter's personality is more introverted, and there's nothing wrong or bad about this. It's hard to watch our children seem unwilling to be social, however, while most of us develop these skills, it comes much more easily for some than others. There's a great book called "The Introvert Advantage" that may be good reading for you sometime in the future if you think this might be part of your daughter's personal makeup. I myself am introverted and while I love teaching preschool and my close friends, *groups* are not my thing at all. I love the one-on-one's with friends over coffee or a drink--this sort of activity fills me up--, but find myself mentally and physically exhausted and drained after any large group activity, even with groups of people I like. :)
Please know that your teachers can gently guide and help your daughter, and PM me if you want some suggestions for that. It's not a guarantee, but I have some games/activities which present opportunities for kids to participate in very safe-feeling activities with other children. Otherwise, give her lots of time, plenty of love, and best of all-- don't talk about it in front of her. That will only serve to concern her and make her more self-concious. Lots of kids go through this stage, and it may take a lot of time in preschool before she is ready to assert her personality!
Best Wishes,
H.