Plural of "daughter in law" [duplicate]
You are right.
When pluralizing words like "-in-law" or "-in-chief", The noun is pluralized, not the modifiers following. Hence:
"Fathers-in-law" not "Father-in-laws"
and
"Commanders-in-chief" not "Commander-in-chiefs"
A handy link is available at http://www.ehow.com/how_5620678_spell-plural-fatherinlaw-similar-words.html.
Always connect the parts of these family nouns with hyphens (-) in both the singular and plural. Thus we write "father-in-law," "sister-in-law" or "brother-in-law." It is not considered correct to write them without hyphens.
Form the plural by adding "s" to the noun, and not to "law." Thus we can write, two "mothers-in-law" and three "sisters-in-law." These are the correct forms even though native speakers may not use them.