Who performed the marriage ceremony between Einstein and his cousin?

Schrodinger performed the ceremony…or maybe not.

@Cruiser Schrodinger was busy killing cats.

@talljasperman Was that a cat or a wedding present in the box?

Probably said by the same person who performed the marriage of FDR to his cousin Eleanor Roosevelt.

Why would it have been stopped? Was marrying one’s cousin illegal at that time?

There are still states where marrying one’s cousin is not illegal (NJ for example). I just googled it yesterday for someone who posted something on FB about a father and daughter wanting to marry. As long as they’re over the age of 18, it’s ok, to this day.

Presumably some Rabbi in Germany or Switzerland. You would have to look at Jewish ecclesiastical rules on marriage to learn about marriage between close relatives.

Marriage between cousins was actually quite common back in the day. And it’s not unheard of even now.