CATHOLIC ANSWERS:
Question: I read recently that the Catholic Church had rejected Mormon baptism, since their view of Christ and the Trinity is so unusual. But I have to ask: Are Mormons considered separated brothers and sisters? While their views are strange to say the least, they are still separated, and we should reach out to them. If we view them as something other than separated, doesn't that exclude ecumenism? I know that many view them as a cult, but aren't cult members separated as well?
Answer: The reason Mormons are not considered separated brethren is not because they aren't "separated" from the Church-they are-but they aren't "brethren" in the sense required.
The phrase separated brethren refers to those who, though separated from full communion with the Catholic Church, have been justified through baptism and are thus brethren in Christ. The Decree on Ecumenism (Unitatis Redintegratio) of Vatican II teaches that "all who have been justified by faith in baptism are members of Christ's body, and have a right to be called Christian, and so are correctly accepted as brothers by the children of the Catholic Church."
Because Mormonism is polytheistic and rejects the Trinity, Mormon baptism is not valid, and Mormons are not considered separated brethren. For the same reason, outreach to them, while certainly a good thing, is not ecumenism, though it can include dialogue and social cooperation as well as efforts to evangelize them.