What did R’ Soloveitchik hold about the famous ruling from Rabbi Goren about the brother and sister that he ruled was not a Jew? Were there any big Rabbi’s who agreed with Rabbi Goren’s ruling? If yes, where can I see it? Why did the Chareidi Rabbi’s come out so strong against this ruing?

Rav Soloveichik agreed with Rav Goren that the children could be allowed to marry. In addition, I know that Rav Goren was honored by Rav Soloveitchik to give public lectures at Yeshiva University, at which Rav Soloveitchik himself listened and participated actively. There was a committee of top-level poskim of the Chief Rabbinate of Israel who affirmed Rav Goren’s decision there, but they explicitly didn’t publicize their names, so as to avoid the inevitable “shaming” which will come from haredi circles. Rav Yosef Eliahu Henkin, undoubtedly one of the leading poskim of his time in America, who was known for his honesty even when it caused him opposition, also agreed with Rav Goren and publicized his opinion at the time. There is a well-known obligation, carried out by just about all “front-line” poskim throughout history, to do anything to find a halachic way to allow mamzerim to marry. This often involves unique and/or original interpretations of halachic sources, which inevitably may often bring controversy.