What horrible people! You must’ve been so shocked.
If you don’t like confrontation, then you don’t have to do anything. You know what they think about you. And I’m sure you now have a pretty low opinion of them for being so nasty.
What happens going forward depends on how close a family member they are, how often you have to see them/contact them and in what context. Could another family member update them on news? Could you keep future contact with them minimal and by email or message? If you have to encounter them at a gathering, could you keep conversation minimal and superficial and give them a wide berth?
If they ask you why you no longer call or contact them much, you could, at that point, say it’s because you overheard what they had to say about you when you last called. And, naturally, it made you disinclined to have much contact.
But they may not ask.
Try not to take it to heart. Hold your head high. People who talk so nastily and judgementally about others are usually not worth the bother. There are many lovely people in the world who I’m sure do/will love and appreciate you ❤️
If you want to avoid further confrontation/conflict, I wouldn’t tell any family members about this or it could escalate. Could you confide in your partner or a friend who doesn’t know them and will never have to meet them. Or book a therapy session if you need to get it off your chest to a real person who will empathise.