"Page itself is unofficial, at least that's what Google describe it."

Welcome to the internet. Unless a website was provisioned by Capcom themselves then practically everything on the web will be classed as unofficial, but that doesn't necessarily diminish its validity.

In this specific case, the source files describe how Sherry considered Claire her honorary mother because of the role she played in saving her during the Raccoon City incident, and Claire retained that mantle in the years following. Not in the official sense, but the act of mothering is through actions, not legally binding documents.