Q. It is said that ancestors are present at the weddings of their descendants. Is there a source for this tradition?
Which are the ancestors that come and do they need an invitation?

A. Osrei Lagefen (p. 143) mentions that the neshamos of the ancestors arrive at the time the brochos under the chupa begin. He adds that the candles lit during the ceremony may be alike the neros neshama lit on days of remembrance in the honor of the neshamos.
Nitei Gavriel (Nissuin 1 p. 20) quotes that the neshamos of the children yet to be born from the couple are also present, and that is the source for the emotion and tears of the chassan and kalla.
He adds (ibid. p. 55) that a week before the wedding it is tradition that the chosson and kalla, as well as their close families, visit the kevarim of their forefathers up to invite them to come to the wedding.
He quotes as the source for the visit of those neshamos the Zohar (Parshas Balak and Pinchas), and up to three prior generations come to the chupa. (ibid. n. 14).
Yalkut Yosef (Chupa Vekiddushin 1: 3: n. 7), explains that one pf the reasons of mechilas avonnos and the forgiving of sins at the chupa, is to honor those visiting neshamos of the forefathers, so they should not be embarrassed by the sins of their children and descendants.
Konnanto Meoz (Beitza p. 71) mentions that this is the source that some communities have to recite a brocho on bessamim and spices under the chupa, similar to havdala for the departing neshama yiseira, that visited us on Shabbos. He also mentions that in some communities they would place an additional chupa for the visiting neshamot.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a