"Crooked House" contains almost no profanity, no sex or nudity, and is bloodless despite it being about a murder. No violence is shown other than a woman who shoots at moles. Sexual activity is discussed (albeit not as salaciously as "Ordeal by Innocence") and alluded to (bare shoulders, kissing and toppling over), but would be considered a light PG-13. The movie is slow and not especially clever in dialogue, but I found the mystery to be compelling enough. I don't know that it would hold much rewatch value considering that it's not the sort of movie where the clues all come together at the end giving you an "aha!" moment -- once the murderer is revealed, it seems painfully obvious.
The greater caution for families with younger children (say 13 and under depending on the individual) is the depiction of the child in the house, who is consistently rude to her authorities and manipulative towards the other characters in disturbing ways. The disinterested adults do not discipline or discourage her behaviors.
Without getting into spoilers, if you choose to watch, be prepared to have some discussion about the themes of justice, revenge, and the difference between true and distorted love (consider the final scene of the movie and the motives behind how it played out).