1

Thought to be cousins, fathers are brothers, both male parents are deceased.

Is it possible to DNA test accurately enough to verify half sibling when female cousin is daughter of brother of my father?

I am male, mother of cousin is also deceased.

Or is there too much DNA overlap due to fathers being brothers to determine authenticity of claim?

2

I think you want to determine whether someone is your 1st cousin or your 1/2 sibling. This is definitely possible. When you are both DNA tested, you will see an amount of shared DNA measured in centimorgans (cM). As you will see in this chart, half-siblings will share at least 1317 cM, while 1st cousins will share 1225 or less.

This assumes that your mothers are unrelated.

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