My wife's cousin has not been to confession for many years. She will be attending Mass with us. What should we do if she presents herself for Communion?
It is not your place to judge for your cousin-in-law whether or not she should receive. You cannot know the state of her soul, and you cannot know whether or not she has been to confession and simply chose not to discuss the matter with you. The only person you are able to examine in such a manner is yourself. Instead of worrying about your cousin receiving, perhaps you might use the time before receiving Communion to examine your own conscience.
After Mass, you might say something like, “I’m so happy that you have returned to the sacraments! Congratulations! When did this occur?” If this opens a conversation in which she admits that she hasn’t been to confession in years, you could gently explain that Catholics must confess mortal sin at least once per year. If not, then perhaps you’ll at least have given her something to think about.