You mean I can’t pray with my mother-in-law (she’s non-denominational)? My wife can’t pray with her mother?
Assuming the prayer in question is orthodox, it most likely falls under the principle of
de minimis non curat lex (
Wikipedia: De minimis). I believe I already mentioned this to Monica. I said there’s probably nothing wrong with saying Grace before meals with her non-Catholic family but she should not attend an Eastern Orthodox Divine Liturgy.
My mother-in-law can’t attend Mass with us when she visits? Or even better; my mother-in-law goes to Mass and I’m the one that has to go to Confession?
Non-Catholics are free to go to Mass, indeed they should be encouraged because it may help them in their conversion. The Mass is divided into the Mass of the Catechumens and the Mass of the Faithful because in the early Church non-Catholics, catechumens and public penitents were not allowed to stay for the Mass of the Faithful (that is, the Eucharistic Sacrifice) because it is preeminently holy. The Divine Liturgy of St. John Chrysostom still maintains an explicit mention of this when the priest dismisses the catechumens, saying,
All ye catechumens, depart! Depart, ye catechumens! All ye that are catechumens, depart! Let no catechumens remain! But let us who are of the faithful, again and again, in peace pray to the Lord.
However, this practice was abandoned several centuries ago and everyone is permitted to stay for the Mass of the Faithful. Therefore, there is nothing wrong with a non-Catholic staying for the whole of the Mass, as long as they are respectful (and, of course, do not receive Communion).
A Catholic cannot attend a non-Catholic service, at least not actively participate in one. There have been certain fuzzy lines with regards to the Eastern Orthodox but that should not give people the impression that it’s a free-for-all even with regard to their Liturgies and certainly not with regard to Protestants. A Protestant public prayer meeting, whether it is held in front of an abortuary or in a “church” building, is the same thing. But short, simple, orthodox, private prayers, especially with the members of one’s own family, is something rather different.