Don’t believe everything you hear. The soy s not believable according to halacha and therefore go to the Chatunah.
The Mishna in Yevamot 117a writes anyone is believed to give testimony about except for her mother in-law, her co wife he husband’s brother’s wife and her stepdaughter.
Rashi explain that the reason is since the women hate her we are concerned they will try and trick her to get married even if her husband is really alive . We see there are five women who are not believed to give testimony about the other’s marriage or lack thereof (for example the husband dies). These women want to trick her into thinking her husband is dead and she will remarry and then when the truth comes to light she will be forbidden to both and her children will be mamazerim. R’ Akiva Eiger writes that there are two ways to explain this . One is that since they hate each other they will mess her up as revenge even if she will no gain anything from this she wants to cause her pain. The second reason she will lie is to help herself to save from a perceived loss of money. For example a mother in law who feels her daughter in-law is wasting her property and her money . As Rashi (Yevamot 117 DH Yevimtah) writes, “she thinks to herself she is going to eat up all of my life’s hard work.” Therefore she tells her daughter in law when her husband is away that she died so she will marry someone else and become forbidden to her husband and therefore not lose her money.
After a lengthy discourse, R’ Akiva Eiger concludes that both of these reasons apply. She is suspect pf lying because she hates her and wants to hurt her. She also might lie even if she doesn’t hate her so much for example the are engaged and she sees she is wasteful.
We see from her that a co-wife is not believed to tell her to mess her up. In our case though the two women are not actually co-wives since one was a proper wife and the other was maybe a Pilegesh. In the Shut Pnei Aryeh (20) however, is of the opinion that even a wife to a pilegesh would apply and she is not believed.
Even though the Chazon Ish ( Even HaEzer 56:4) argues against the ruling of the Pnei Aryeh, he only argues in that specific situation. There the second women were forbidden to the husband prior. In our she was not forbidden to him and the Chazon Ish would agree they are co-wives and she is not believed.
Even more so in our case the original wife is to believe to say that other woman was married, that needs real testimony and witnesses to establish that.
As the Shulchan Aruch (Even HaEzer 152:6) A woman who comes and says I am married and divorced she is believed because she created the fact that she was potentially forbidden and she may cancel it by saying she is not forbidden. Some say it is only if she says she is divorced immediate after she sys she was married, unless she gives an excuse why she waited. The Shulchan Aruch adds in Seif 7: If there were witnesses to the fac that she was married then she is not believed to say she is divorced and if she gets married to someone else we force them to get divorced.
So any women who two witnesses say she was married she is not believed to say she is now divorced without other witnesses corroborating that fact. The Bet Shmuel (11) writes that :We see from here that one witness is not believed to say she was married.” We need two witnesses who are halachically approved to be witnesses to say she is married, not one witness. Even more so in our situation one woman can’t testify that another was married.
Another thing is that even I we could believe her that she was married, it would only work in as much as saying the mother was married at that time and transgressed a prohibited relation. She will not be believed to make a child into a Mamzer. Only two witnesses have the power to do that. Even the mother herself into is not believed that her child is a mamzer only that they did a forbidden act. Look in the Shulchan Aruch Even HaEzer 4:29 that she is not believed, evermore so someone else.
You are correct that if the Chatunah was actually a forbidden union, she was a actually a mamzeret, then it would be forbidden to go the wedding. Just as the Yalkut Yosef writes in Hilchot Kibud Av Vem (vol. 2 pg. 256 halch56) it is forbidden to go to a wedding with a Reform Rabbi performs a wedding it is not according proper Jewish custom (Kedat Moshe V; Yisrael.) So too any wedding which is forbidden in halacha it forbidden to go to the wedding. Our situation it is only rumors which are not true and you may go and enjoy the weeding.