As far as we know, Michelle hasn't told Sharon about her one night stand with Grant and who Mark's father is.
I don't believe she would either for fear of Grant finding out. If she was ever going to do it, she would have before now. Michelle isn't/wasn't stupid. She was young and naïve when she told Den about Vicky, and chose not to repeat that mistake when she backed out of telling Grant in 1995. Plus, Den and Grant are two different people. Den was happy having Jan and Michelle/Vicky at arm's length so he could keep up pretences with Angie.
Sharon went berserk over Den. I don't think that alone would put Michelle off saying anything as she loves Sharon, but knowing what Grant and the Mitchells are like would ensure her continued silence. Michelle was always a determined person and I don't believe she'd backtrack on the decision she made when she left, especially after so long.
If this is the secret/contents of the letter then it's totally unlike Michelle. I imagine they will say something ridiculous like Michelle thought Peggy should know before she dies. Except Peggy and Michelle didn't even like each other. Peggy could have died at any time of old age, Michelle has never felt the inclination to tell her before.