Blood is thicker than water?

She said it to allay her fears that things between two members

of her family would be as they once were.

You see the younger of the two in question had, in the eyes of the elder,

committed one of the worst crimes in a family sense...

he had abused her and her partners trust.

Needless to say the elder, although was trying her best to recover and forgive him, knew that things between them would probably never be the same again.

But, back to my quandary. It wasn't the first time I had heard someone quote these words, I have been witness to this statement on many an occasion and can remember as a child trying to fathom what this is supposed to mean?

Officially the saying is of German origin, a German proverb originally:

Blut ist dicker als wasser.

Which in English speaking countries generally means that the bonds 

of family and common ancestry are stronger than those bonds

between unrelated people.

So am I to interpret from this statement that because the third party in this situation is not related to her by blood she believes that the tie between the younger and elder to be stronger than that of the elder and her partner? Surely this cannot be?

What I find all the more disturbing is the person using this statement is indeed part of a family in which there are several 'step' relationships. Ipso facto is she saying her relationship to the non blood related members of her family are less important or strong than the blood relations?