She'll just have to get over it. I agree with the others - where does she think her grandchildren came from
I feel for you though, I've had it happen twice. Once the first time I ever met ex H's mum. Yeah, that went down well... She literally walked in and yelled "OMG ARE YOU HAVING S3X?!" O.o
And the second time the first day DH and I moved into our house - he had just flown home from work too and his Dad rocked up without calling first and knocked on the door (which is right next to our bedroom). We didn't hear him for a good 10 minutes before we realised! He didn't say anything but was all miffed about it and apparently went around telling the rest of the family! After DH's uncle told him it was his own fault for not asking if he could come over, he shut up. He's over it now but we never mentioned it to him!