Legal Question in Criminal Law in Missouri

Technically it may be theft to remove flowers that are placed on a grave by someone else. However, it is theft from your Brother's estate. Therefore, as wife, she is the person most fit to bring a civil action for theft. From a civil standpoint, the cemetary probably has retained the right to mow and remove temporary objects (trash, flowers, cards) from around the grave markers and stones.. They may be able to direct the owner(s) of the plot to do that. As the widow, your sister-in-law may have the authority to do that.

But, if doing so, one is trying to cause emotional distress to the people that left the flowers, that person might be subject to a civil suit from the flower placer(s). In that sense, you and your mother may have a civil action against your sister-in-law for infliction of emotional distress.

Good luck