sister in laws or sisters in law Your spouse’s female siblings are not your sister-in-laws, but your sisters-in-law. The same pattern applies to brothers-in-law, fathers-in-law, and mothers-in-law. Facebook Twitter Google + Do you make these 10 mistakes? stood vs stayed ruff vs rough appauled vs appalled wreckless vs reckless greatful vs grateful woman vs women epic vs epoch ceasar vs caesar heal vs heel benefactor vs beneficiary