Could somebody attend his non-Jewish relatives' wedding ceremony if they are held, let's say, in a church or a Buddhist temple?
See the linked answers. Let's say "not recommended", put mildly.
I don't know the nature of this non-Jewish relative connection, that's its own set of issues.
But in a nutshell: Judaism traditionally prescribes staying far, far away from anything that looks like non-monotheistic worship or a house thereof. There would be very little issue, as far as I know, of attending a friend's Muslim or Bahai wedding, for instance. Rabbi Yehuda Herzl Henkin allows one to enter a Unitarian church, though I doubt attending a worship service there is a good idea. (I don't know how much of a "worship service" the wedding entails.) Moving along the spectrum of Christian denominations towards Catholicism and then Eastern Orthodox, where there are more and more "physical symbols", the issue becomes increasingly thorny.
My understanding is that rabbis assume Hinduism is given the same treatment that Greek paganism was in the Talmud; it's debatable whether Buddhism is viewed by Judaism as "paganism" or "atheism."
As a convert, this has been a bone of controversy in my family. My Rav, Rabbi Gedaliah Anemer, zt"l, said it was forbidden to enter the sanctuary of a church. Rabbi Maurice Lamm, in his book Becoming A Jew, also does not allow any leniency.
When my father died, I brought up the issue because I was asked to speak at the memorial service. Another Orthodox rabbi, Rabbi Bertram Leff, shlita, suggested that I do it like a kohen in a different room other than the sanctuary; I rejected that because it would be unexplainable to the non-Jews, and my mother would have been embarrassed. Holding firm to my position, my mother agreed to hold the service in the non-denominational chapel at the funeral home, which just happened to be next door to my parents' church, and I agreed to go to the reception afterwards in the church's social hall because it was in a separate building than the sanctuary.
But I have seen examples of Orthodox rabbis who did go to churches. The Chief Rabbi of England attended the wedding of Prince Charles and Diana in Westminster Abbey, but that was permitted on the basis of kavod hamelek (honor to the king). A more troubling issue has been the case of Orthodox Rabbi Yechiel Eckstein, founder and president of the International Fellowship of Christians and Jews. Rabbi Eckstein's visits to churches has raised millions of dollars for Jewish social services in Israel. But I feared that if my mother knew of him, she would want to know why I couldn't have her funeral in her church.
I wrote to Rabbi Eckstein by e-mail, told him of my dilemma, and asked on whose heter did he rely upon to go into churches. Thinking I was asking him for permission, he first said, "I don't poskin shailos" (i.e. he doesn't decide legal issues). But pushed to answer my question, he admitted that he was relying on his own, unprecedented, legal opinion because what he is doing is good for Israel. I am glad my mother doesn't watch Christian television shows.