Can you block your oldest daughter on Facebook? I believe if you actively block someone (as distinct from not being friends with them), you don't see any of their activity, even if they tag someone you are friends with. You could also politely ask your cousins not to tell you about things that they see from your daughter on Facebook unless there is really an urgent need (e.g. someone's hair is on fire) because knowing about them is stressful. This sounds like a situation in which the less you know about your oldest daughter's social media escapades, the better your emotional health will be.