I think you should tell your WW about it. If you WW was already confiding issues in her mother, then she has already brought your mother into it - you didn't tell her mother out of spite, you told her beacuse you thought she already knew. Your WW will hopefully understand that.
I told two people after dday outside of my best friend and own mother - one was a coworker who had been through a similar situation and was the only person I knew empathetic to my actual work situation in trying to deal with an affair and work. The other was a playdate mom, and I did *not* wan tot ell her and still regret her finding out - but she was concerned on my absence from everything for a few weeks, I'm a piss poor liar, and she was concerned with a whole bunch of life changes going on. I wound up full on bawling when she asked me about why I didn't seem to know when I was scheduling my daughter for swim lessons I told my husband that I told both of them and he was understanding. He said while he'd rather the playdate mom not know, it's his screw up, not mine, and he hates that I've been put in this "plaster a fake smile on my face" type situation and he will take repercussions of what he's done.
My MIL knows. It was one of my conditions. I think, depending on the parents, it can be useful to have the WS's mother or sibling know if that person is supportive of the WS, the BS, and of the marriage and of any choices that the two people are making. A parent will always love their child even if they're disappointed in them, and sometimes a WS needs that kind of support system. Of course, not every parent is actually helpful to tell.