While we can hope that your priest was trying to find a way to accept the fact of his siblings’ defection from the Church, his statement is extremely problematic because it demonstrates ignorance of the severity of the gulf between Catholicism and Mormonism.
Mormonism is not a Protestant “brand” of Christianity; it is, in effect, the rejection of Christianity disguised with Christian terminology. Mormonism believes in many gods, it believes that a man can become a god (which is, incidentally, the devil’s first suggestion to Adam and Eve
[cf. **Gen. 3:4-5]), and it has many other beliefs that are entirely incompatible with Christianity. For more information, I recommend the articles and books listed below.
Gently point out to your priest that he cannot hope to help his family understand truth – which ultimately points us to Jesus Christ, who is the Way, the Truth, and the Life – unless he is willing to understand what Mormonism is and help his family to understand that they have accepted a non-Christian religion. Indeed, some have argued that Christian converts to Mormonism have accepted an apostate religion. As a brother, and as a priest, he must be willing to explain to them the seriousness of what they have done. And, even if he is unwilling to do that, he should at least refrain from appearing to make Mormonism seem preferable to Catholicism to his own congregation.
Non-Catholic Groups** (scroll to sub-head “Non-Christian”)
Inside Mormonism by Isaiah Bennett
When Mormons Call by Isaiah Bennett
Search and Rescue by Patrick Madrid