Dear Jewish Fairy Godmother:
My son is graduating high school next week. My mother and brother
live in town and my sister lives two hours away. My brother and sister
haven’t spoken in two years. That’s almost an improvement from when
they did talk, because it was always conducted at several decibels
above normal peaking volume and accompanied by loud cursing,
insults, and snide remarks. None of the rest of us is anything but
disgusted by their behavior but no one has been able to convince them
that they should just act like grownups, not ten-year olds. My sister
has said she wants to come, but she’s said that before and not shown
up. How can I prevent them from turning my son’s graduation into
their circus?
Tired of Peacemaking

 
Dear Tired/Peacemaker:
Clearly they’re more attached to the dysfunction than they are to the
idea of happiness and family harmony. Clearly also both of them value
you, and clearly both of them have some sense of family or they
wouldn’t want to be with your nuclear family. But you do have some
leverage. Have the same conversation with each of them. It goes
something like this:
You’re my sibling and I love you, even though you’re not always the
easiest person to be around. I’ve tried to talk to you about this issue
before and you haven’t listened. Listen up because I am deadly
serious: If you screw up my son’s graduation I will shun you for one
year. I will not speak to you, I will not text or email you, I will not
invite you to family events or say yes to any invitation o extend. I
expect you to be civil, polite, and act like an adult. You can talk about
the weather and you can talk about my son, praising and
congratulating, and you can compliment the food. You may not discuss
anything else. And if you raise your voice above conversational level.
If you violate any of these terms I will insist that you leave.

Get your mother to back you up. Then stick to your plan.