Not Twins

[Jimmy Higgins]

You are introduced to two 5-year-old girls. They look so much alike that you ask, and ascertain the following:
> They have the same January birthday in 2009
> They share the same biological mother and father
> They are not twins. Not identical twins, not fraternal twins

I hate logic questions that are asked poorly. Doesn't specifically call out this is the biological father.
And if you are asking questions, you'd hope the parents weren't so obtuse to say oh they aren't twins... and forget to finish the thought.