WR, I think she was just trying to find a way to jump into the conversation and let you know that she was reading along. No where did I see you claiming the recipe as your own, in fact, it may be one that your MIL uses, but it's a French recipe that she has claimed as hers. LOL! BTW, you didn't need to justify why you were discussing the recipe to her. If it's brought up again, find the recipe and state which cookbook you got it from. That should cool her jets about who is claiming what. LOL! I wouldn't let that little bit of conversation rattle me.
They are reading everything you and your sons are doing, if they have access to it. They have heard so much BS from your h that they are curious to see what you really are doing. You should be flattered that you are taking up so much room in their heads. When people are like this, I have as little contact w/them as possible and that includes FB time.
My advice, don't put anything out there that you don't want them to know about right now.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.