Here are your relevant Fatwa's.
"and call to witness two just ones from among you and give upright testimony for Allah (Talaq-65-2)."
Again it is said
"You who believe, call to witness between you when death draws nigh to one of you, at the time of making the will, two just persons among you or two others than among you (Almaida-5-106)."
This would mean that the witnesses must be Adil i.e. the just ones as laid down in the Quran. The Sharia does not make admissible the evidence of persons convicted of any moral offence or persons having reputation of immoral character or persons who have any bias for or against the persons about whom the evidence is to be given.(Fatawa-i-Alamgiri Arabic, Volume 3, Page 465).
Then the verdict is also as simple as that, the witnesses were incoherent, the witnesses were unreliable, sufficient evidence could not be collected. Case closed.