Why do you think there haven't been siblings for the other girls in actual doll form?

Actually I misspoke! Nellie is not only the best friend to Samantha, but also her sister! Adopted siblings are valid and loved. 

So I reckon that siblings weren’t ever produced simply because of the size difference. Younger and older siblings would need to be smaller and larger, and that would be way too much work for AG to put into producing. It’s a whole new face mold, wig size, body size, limb size…. just not worth it! Wellies work great as younger siblings, though, and also stand alone as their own characters, so it all worked out well in that case.