DB's mother is trash-talking my MB

[Anonymous]

My MB and DB's mother have always had a complicated relationship. It has gotten much worse since my charge was born.
Grandma stops by to see him once or twice per week. I get along very well with her, so I welcome the visits.
Lately, she has been saying some awful things about my MB to me. She complains that MB is too strict about his sleep schedule (this schedule has been amazing!!) and that she's trying to keep my charge away from her. I try not to say anything, just to let her vent. It's happening almost every time she comes over. How the heck do I address this? MB is pregnant and very emotional. I don't want to add more to her plate.
I don't want to have to tell DB anything about his mother. I'm concerned that although we have a fantastic working relationship, family is family. I don't want to piss anyone off. Any suggestions?
Please be nice. I've been a nanny for many years. I've just never been in this situation.

[Anonymous]

Just keep on what you're doing. Let grandma vent, chime in with positive things about how great the sleep schedule is or whatever, and keep your mouth shut to your employers unless something ges said that genuinely needs attention.

[Anonymous]

MB here. That's awful - I'm sorry you're being put in this position.

If possible, work up a few lines to say to the grandmother.

Mrs. X - I'm sorry but Jane and John are my bosses - I'm not comfortable having these conversations with you.

I actually think Jane is a great mom and I enjoy working here. It's really hard for me to hear you talk that way - perhaps we should just steer clear of this topic.

I'm sorry you feel that way but they are my employers. I think you need to talk to your son.

Etc...

If you can't have that kind of conversation with her then you talk to DB. He needs to get his mother under control. You could just say that it seems like his mother has some issues with his wife (this can't possible be a shock to him) and it makes you uncomfortable. You've asked his mother not to discuss it with you and you really love your job and both of your bosses and don't feel comfortable talking about MB behind her back. But you don't want to make anything worse. So if grandmother can't stay off the subject then you'd prefer not to have her visit while you're working.