Are they real sisters?

I feel bad for asking, but I really want to know. I can see the resemblance between the brunette and "Mommy" but not with the blonde one. Plus, I've only ever heard her call the mom "Mommy" once and they don't really interact like a real mother/daughter would. That all leads me to believe that maybe she's not really in the family and that they're only scripted as "sisters".

Are they 100% blood sisters, are they half sisters or is the blonde one adopted?