ExP and I broke a good 18 months or so ago. His family were amazingly supportive, and completely on my side, knowing the reasons. We still get on very well, and I actually still get on okay with exP, for DSs sake.
ExPs grandmother is 99. She's quite deaf (well, she declares she's fine, and that everyone mumbles 
) but aside from that is entirely on the ball. She's in a care home, and we only see her a few times a year, as we live a few hours away. I send her letters and photos of DS.
ExPs step-sister broke off her engagement not long before we split up, and the grandmother was distraught. So when we split, we all decided that it was better not to tell her, because a) she didn't need the upset, and b) we only see her a few times a year face-to-face, exP doesn't always come up when DS and I go to his parents, so it was only a small lie.
However... We saw her yesterday, and she was as usual delighted to see us, gave DS lots of hugs etc. She then also gave me a big talk about how pleased she was we were still a family, and how sad she was about step-sister still. She looked at me and said, "You are happy, aren't you?". Also told me what a lovely girl I am, and how she is so pleased exP has someone like me. And that she prays for us.
I felt terrible! I told her I was very happy, and that DS is such a delight, how could I not be.
Today I still feel so bad for lying to her, but feel that at 99 she shouldn't have to worry about our relationship, when hers with DS (and with me for that matter, I contact her more than exP does!) won't change because his father and I are no longer together.
Am I doing the right thing here?