JOEL - The only sinless person whoever lived on this planet was Jesus Christ. Our goal in this life is to continuously improve ourselves towards perfection as Jesus admonished us to do in the scriptures (Matt 5:48). No one can really reach complete perfection in this life, but as long as we are striving for that goal, we can take comfort that we are on our way towards eternal life with God in Heaven. And even though we cannot live a completely sinless life, we can always repent of the sins we commit and God will remember them no more (Acts 3:19).
As you might notice by the name of our Church (The Church of Jesus Christ of Latter-day Saints) we believe that everyone who joins with God's kingdom on earth is called a Saint, the same as the Church members at the time of Christ. (Rom 1:7, Eph 2:19, Col 1:2). The LDS Church has no "patron saints" and does not canonize or venerate the dead. The word "Saint" is used over 350 times in the New Testament and was never used to described anyone other than the regular members of Christ's Church.
Members of the LDS Church have great respect and admiration for people like Mother Theresa and the work of peace and healing they do throughout the world, but don't regard her as a "Saint" in the same way the Catholics do. One dictionary deffinition of the word "saint" is, "An extremely virtuous person". Mother Theresa would certainly represent this deffinition of the word "saint".