You have received many good answers from good people. Our garments represent the covenants we have made with God in the temple. We wear them day and night as a reminder of those covenants and believe that they offer some spiritual protection by way of that reminder from the temptations of Satan. We respect them for what they represent and thus do not treat them the same as just any dirty laundry. We remove them for normal activities like bathing, swimming, sports and intimacy. Originally, they were not removed for the last item but that policy has been moderated for practicality. The envelope is merely a pocket of cloth that holds our temple robes when we enter the temple for the performances of vicarious ordinances for our ancestors and for eternal sealings to our families. There is nothing special about the envelope. There is sacred significance to the robes inside the envelope. Thus, the whole packet is safeguarded to keep it sacred.
What I want most to focus on is the idea of eternal progression. We were with the Father before mortality and will ultimately return to the Father. However, our eternal reward will be determined in part by our own level of righteousness, obedience to God's teachings and commandments. If we are faithful throughout our lives to the teachings of the Gospel of Christ, we can inherit with the Savior, the Kingdom of the Father. That means that ultimately, we can become like Him and have eternal families. We will progress to become like our Father, spiritually, in the same way that we hope our mortal children can grow up to become like their parents, assuming they have been given a good example by their parents. God wants us to grow up to become like Him. He has given us that promise. Eventually, if we live worthy, we may have spirit children of our own and create worlds upon which they may dwell and develop just as we are doing now. As man is, God once was. As God is, man may become. That is the essence of eternal progression. This is not heresy, but sound doctrine alluded to in the Bible as well as modern revelation.
Lastly, The Church of Jesus Christ of Latter-day Saints is not a cult. It embodies the teachings of Jesus Christ in their fullness. Those that disparage the Church are ignorant of the doctrines and their significance. With the benefit of modern revelation, living prophets and apostles, together with personal revelation through the Holy Ghost to discern truth from falsehood, we know the validity of the doctrines, in full accord with Biblical teachings. Unfortunately, it is mainstream Christianity that is not reaping the benefits of the Gospel as taught by the Savior and His apostles of old.
Try to ask your brother and sister-in-law, with an open mind, to explain the doctrines to you. This will force them to do some studying as well.