Word or phrase to describe relation beween yourself and a step-parent's ex-spouse?

Is there a word or simple phrase to describe the relationship between yourself and a step-parent's ex-spouse? Particularly, I'm seeking to address the case when the ex-spouse is a parent to your step-sibling.

Example: Bob’s mother re-married to Alice’s father. How should one appropriately refer to the relation between Bob, and Alice’s mother?

I’m trying to find something that’s appropriate for a relationship closer than “Bob’s step-sister’s mother” or “Bob’s step-father’s ex-wife” but not quite so close as to just call her “Bob’s mom”.

Solution 1:

There is good Yiddish word for this, mishbucha, which according to my Jewish friends, means someone who is related to you, usually in a complicated way.

Solution 2:

I think I would call her "my stepsister's mother", since that is the relationship that is important to you. Also, anyone who takes a moment to think would figure out that you are not talking about your own mother OR stepmother, since you would have said so in the first place.