Why does the woman's brother-in-law bathe the groom?

There are national wedding customs everywhere, especially ethnic minorities. Although some are difficult to understand, there is always a reason for its existence. So the married father-in-law should bathe the groom's official. Although there is no way to study it, it must have its profound meaning, which will bring the relationship between the father-in-law and the groom's official closer. Probably because of local customs. Every place has its own customs. Do as Romans do in Rome. He is the son-in-law who comes to the door, afraid of bringing bad luck and disinfecting him. The first time I heard my brother-in-law and uncle taking a bath by themselves. If you have this custom there, observe it. Just don't break the custom. Answer finished, please adopt, thank you!