I grew up in a WELS church but now am at a Missouri Synod church. I am told by the church I grew up in that I cannot sing at my mother's funeral. She was a life-long member. What is the thought process here? I am just being told no - that is the way it is.
May the risen Lord bring you comfort and strength by assuring you that those die in the Lord are forevermore blessed (Revelation 14:13).
There is, of course, no Bible passage that addresses your question specifically. That is, there is no Bible passage that states specifically who can and who cannot sing at a funeral service. What the Bible does present are broad principles of fellowship that we then need to apply to specific situations like funerals and weddings, and worship services in general.
The Bible does encourage us to work together with those who have a common faith, and to work together to promote the truth (3 John 8). At the same time God, through the Bible, tells us to separate from and not join in fellowship activities like worship with those who are not one in faith with us (Romans 16:17; Titus 3:10; 2 John 10-11).
In the situation you describe, the difficulty is that you have membership in a congregation of a synod with which WELS is not in fellowship. Your prior membership in a WELS congregation or the family connection to the funeral service does not override biblical fellowship principles.
When you joined a congregation of the Lutheran Church—Missouri Synod, you became part of a church body that is not in fellowship with WELS. As your synod and WELS are not in fellowship with one another, it is not possible for you to take a leadership role in a worship service in a WELS congregation.
This is not an indictment of your personal faith. Whenever I answer questions like this, I try to emphasize the difference between visible churches and the invisible church, the Holy Christian Church. WELS and LCMS congregations are visible churches. If an LCMS member is not able to sing a song at a funeral or receive Communion in a WELS congregation, in no way are we saying that the LCMS member is not a Christian, nor are we pretending to read what is in the individual’s heart. We are happy when a person’s sincere confession of Christian faith identifies him/her as a member of the Holy Christian Church, the invisible church. But only God knows who belongs to that Church; you and I operate in the realm of visible churches. So, while we may have a common membership in the Holy Christian Church with other Christians, their membership in a visible church outside our fellowship prevents us from doing the things we might like—like singing at a funeral service.
You may or may not be aware that representatives of WELS and LCMS, along with the Evangelical Lutheran Synod, have had informal discussions in the past several years to clarify where there is and is not doctrinal agreement. This link will take you to a recent report on those discussions. The report closes with these thoughts: “Perhaps God may guide us to a reestablishment of fellowship at some point in the future, a goal for which we pray and work. But even if we are not able to practice church fellowship, we have found benefit in talking together about church work, in patiently trying to understand the issues better, and in providing a measure of encouragement in our lives of repentance and fidelity to Scripture.” Again, may Jesus, “the resurrection and the life” (John 11:25), bring you comfort and strength through his gospel.