Lucy had it first, Ethel had it last, Mary Lungel had it twice until she married Peter Stern and never had it again. What was it?

“Love”

While love may be an important aspect of relationships, it does not fit the criteria of being something that Lucy had first, Ethel had last, and Mary had twice.

“Luck”

Luck does not correspond to the names mentioned in the riddle and does not explain the transition of having it first, last, or twice.

“Letters”

The answer is specifically about the letter 'L' and not the general concept of letters, which does not address the specific clues given in the riddle.