Just write what your honestly thinking and if that person loves you they will get over it and know which subjects are bugging you. Maybe they need to hear it.
I wouldn't waste my time with a mean relative. Your letter is not going to make them nicer, and will probably just give them one more thing to be mean about. Do you need to tell them something specific? If so, please restate your question.
You show your manners and thank for a gift no matter how small. Perhaps if you paid the relative a bit more attention they would not be quite as ungenerous. Sometimes people have debts that you do not appreciate - also if there is a large family it can make it very expensive.