OP, it sounds like you and MB are close and she feels comfortable confiding in you.
I don't agree with those saying OP should mind her business because who knows the situation and the mom obviously trusts you and you may be the only one she's comfortable coming to.
Just avoid shooting dirty looks because it does nothing and you don't want DB catching you doing it and having drama on your end.
Being a nanny is not only professional but it can get very personal. Things happen and you cant just turn off that emotional switch. I think if you feel close enough with your MB and don't mind her speaking to you about this then thats great and you should.