How to dna test a persons family that is not willing?

I just found out my father passed in 89. I was born in 84. I'm now twenty six...My family is just now telling me, my real father committed suicide. The decease's mother is 87, she is saying why bother, leave it be. If i had a good father that was good to me ...I wouldn't bother. But I don't ..I need answers. So I know. So I need to dna test them. What should I do if there not willing?How to dna test a persons family that is not willing?
If they don't want to know then there is nothing you can do about it.

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