What about going to confession as a family, including your mother-in-law? I have several friends who make weekly confession a family thing and then they'll all go out to dinner afterward. They definitely make it something special. Confessing our sins *is* a difficult thing (REALLY difficult for some people if they've gone a long time without confession), so rather than a "hard nudge", I would suggest compassion and realizing that sometimes the first step is the hardest one. Suggesting that you all go to confession together may help persuade her to go!