The Catechism says of charisms (like prophecy) that:
- "Charisms are to be accepted with gratitude by the person who receives them and by all members of the Church as well. They are a wonderfully rich grace for the apostolic vitality and for the holiness of the entire Body of Christ, provided they really are genuine gifts of the Holy Spirit and are used in full conformity with authentic promptings of this same Spirit, that is, in keeping with charity, the true measure of all charisms.[Cf. 1 Cor 13 .] "
This understanding comes from St. Paul’s letter to the Corinthians (ch. 13), where he says one could make all the prophecies in the world, but it means nothing without love. If this alleged prophecy is causing your family to set your son above his siblings, then it seems to me that it may not be in keeping with charity. In any case, I think you could gently remind your husband’s family of St. Paul’s admonition, and just let the fervor over the prophecy die down.
I also wanted to say that attending an EF Mass or being “traditional” does not guarantee that one won’t run across superstitions and/or “free-form” spiritual practices. In fact, one may come across them just as much, but they take different forms. My grandmother, God bless her, is a very traditional Catholic who always attends Mass in Latin if she can. For years, she believed it was perfectly okay to worship the Virgin Mary, and that she was on the same level as the Holy Spirit. I’m still not sure she’s convinced.
Finally, just as an aside, you don’t have to know Latin to benefit from a Latin Mass! I don’t, and I love the one we have in town every other week or so–they’re so beautiful! There will usually be a missal-type booklet with translations for you, so you can start to learn, and the readings and the homily are in English. It’s actually become a sort of evangelistic tool around my university–all the Latin students, Catholic or not, love to come and hear it spoken aloud.