Q. If after a chupa, it became known that the witnesses were related to each other with a disqualifying familial closeness, does Rav Miller hold that the kidushin have to be repeated, or we can bedieved relay on the Chassam Sofer that permits?
If they have to be repeated, do you make a brocho and repeat the sheva brochos too?

A. Chasam Sofer (Tesh. E. H. 1: 100) indeed rules in a similar case (one of the witnesses was related to the bride) that the kidushin take effect. He explains that since there where many other people present, who would qualify as kosher witnesses and it is an “umdena demuchach” (obvious case) that the couple under the chupa got married, we all became de facto witnesses (anan sahadi) that the marriage took place, and there is no need to repeat the act of kidushin.
Otzar Haposkim (42: 31) quotes diverse opinions in regards to the above. Mentioned there is also the tenet of Kerem Shlomo (21) that it makes a difference whether the chosson himself nominated the eidim or when they were chosen and dedicated by someone else, like the ministering rabbi. In the later case, even if the chosen witnesses turned up to be non acceptable, since the chosson had in mind only the proper eidim, the selection becomes ineffective and anyone kosher to witness, will become automatically an eid.
Hanesuim Kehilchoson (1: 8: n.15) deduces from the above, that it is commendable that the officiating Rov should be the one to appoint the witnesses. However, Nitey Gavriel, (Nisuin 1: 20: 5: n. 8) quotes different opinions and mentions that the minhag Brisk is that the chosson nominates the eidim.
Horav Shlomo Miller’s Shlit’a opinion is that it is better when possible to tactfully and discreetly, repeat the kidushin without any brocho at all. However, as far as a get is concerned, she is considered married without doubt.

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