Of course, this "egg mom" doesn't think of herself as the real mother of these girls - and of course, in our relativist, subjectivist world, that would indeed seem to "make it so," as Captain Picard used to say.
Except, there is a little glitch. Biology says that the "egg mom" is in fact the MOTHER, the ONLY one, regardless of what she may think of that or whether or not she agrees with it. Biology doesn't ask for personal opinions, and doesn't change based on social trends, opinion polls or political correctness.
And the girls may in time agree. They can't be expected to necessarily stick to the neat little story their "co-parents" have invented. Many donor-created children don't necessarily agree that their mother's egg is just a helpful little insertion into their social parents' reproduction - they know very well that the egg is identified with the person whose name in the real world is always "mother." They know very well that they have been half-adopted, and that they are being raised by a woman who is, in fact, a stranger. To quote "Son of a Surrogate", who explains how such children may feel:
What do we think about what you think? What you think doesn’t even make sense to most of us. It doesn’t make sense to the majority of people...Do you expect us to have this sort of delusional thinking that you do or do you expect us to think like 99.9% of the general population...Read the article here.