Most upvoted comment
My great grandfather gave Einstein violin lessons(r/pics)
The guy standing next to him is David Rothman. David Rothman owned a department store in Southold (which is still there) and that’s almost certainly where the sandal incident happened.
Rothman and Einstein were in an informal quartet together. Milton Samuel, your relative, was the viola player.
P.S. On that occasion, they only had women’s sandals in stock and Einstein bought them and wore them all summer. You can see him sporting them in the two photos in the link!
P.P.S. Here is a book about those times.