Legal Question in Family Law in California

This is an interesting question as grandparents have been successful over the past dozen years gaining court recognized visitation rights. If the mother, herself, has been absent as you say, there might be more sympathy for the GP. Courts want family visits so this might be considered a substitute for the absent mom. Ordinarily, it would be unusual for a GP to go to the trouble of seeking court relief when visits are ongoing and adequate. You have to ask: what more does GP want? Is this a step to be legally recognized in order to gain complete custody and control from you, a "mere" step parent? So, standing is possible depending upon all circumstances.