I'm related to David, he's my grandad.
Estoy relacionado con David, él es mi abuelo.
Why is estoy used and not soy?
It seems to me that the relationship is permanent and defining, so I would expect soy to be used.
As Javi mentioned, a better translation is:
When to use
That said: what about inlaws? That relationship is not necessarily permanent. Ask Kim Kardashian.
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As comments state, permanence is not always the key to choose between ser and estar.
When combined with the past participle of a verb, "ser" creates a simple passive, "estar" expresses the state.
This should explain why you need "estoy" in your example.