Albert Einstein’s second wife was his first cousin on maternal side(mothers were sisters) and 2nd cousins on fathers’ sides (fathers were first cousins). No children from that marriage.
My paternal grandparents were third cousins, once removed, and more distantly related on other lines—they were from the same small village where the majority of marriages were between two people from the village (the rest from other villages within 15 miles). When you get to second or third cousins the percentage of shared DNA is much less. I think second cousin marriages were common historically among European Jews (but I don’t know what percentage of all marriages they were).