Prove identity by blood samples?

You are medical officer of an intervening force that is on a stabilization mission in a foreign country.

A wounded local civilian is brought to you at the field hospital, which is located in one of the camps of your forces.

Soon, it is suspected that the wounded is the brother of two soldiers from the local allied forces that have carried out insider attacks against some of your troops a few days before.

It is proposed to take blood samples in order to confirm whether the wounded person really is the brother in order to, if positive, interrogate him about the plans and location of his brothers.