This topic is brought up because kids and elders are using them wrongly--even the MEJKOOB LAUS OF AMERICA ARE USING THEM WRONGLY TOO.
First and foremost, there is no such thing as Niam Tais Ntsuab. This term is probably a mispronunciati on of the correct term--Niam Txais Ntsuab. Mispronunciati on of words such as this is similar to some people calling "roob" as "tsoob" or "riam" as "tsiam." Mispronunciati on is not the issue; but, the fact that when they are corrected, they would say something like "peb yeej ib txim hais li no" or "peb cov niam txiv yeej qhia peb li no." This is just plain ignorance.
Niam Ua Luag is translated as "bridesmaid" in the mong dicionary. This should not the case at all. The English dictionary defines bridesmaid as "the female who accompanies the bride on her wedding day." The Niam Ua Luag in Mong cultural weddings does not accompany the bride during the wedding. Her role is strictly to look after the bride to make sure she does not do extra curriculum activities with other guys during her wedding. I do not believe Niam Ua Luag and bridesmaid have the same concepts and implications. The correct term can be "chaperon."
Going back to topic, the term "Niam Ua Luag" for Mong, is defined as "the sister of the groom, to chaperon the bride during her wedding." Most importantly, Mong can only refer to the sister as "Niam Ua Luag" if the wedding is done through the "lawv vauv" process. In this case, the sister can never be called "NIAM TXAIS NTSUAB."
Niam Txais Ntsuab--this person is actually referring to a couple (but the husband always sits on the table). The complete term is called "NIAM TXAIS NTSUAB, TXIV TXAIS NTSUAB (for some reason unknown to me, we always refer to the wife first...such as Niam dablaug txiv dablaug)." This couple is, of course, from the groom's side. However, if there is a "NIAM TXAIS NTSUAB, TXIV TXAIS NTSUAB," then there must be a "NIAM XA, TXIV XA" on the bride's side too. These two couples only exists during NIAM TSHOOB (not your regular wedding we see in today's world). In basic terms, these are the "Receiving and Delivering Couples."
With this in mind, there is no such thing as a Niam Txais ntsuab in a regular hmong wedding--only a Niam Ua Luag. Likewise, there is no such thing as "Niam Ua Luag" during a "Niam Tshoob"--ONLY A NIAM TXAIS NTSUAB.
Good luck.