Linda,
As difficult as it may be, you're going to have to find a way to break from these family members. Your first responsibility is, and must remain, to your son and yourself.
Something that might make it easier for you to do is to explore social services in your community that would provide transportation to your aunt and uncle. That way, you could give them alternatives when you tell them that you can no longer continue to provide them with transportation. Check for a senior citizen's center in your area. They're often good sources of services and information about other groups that provide services.
Keep us posted?
Good luck,
Teri