Can I marry my Fathers brothers daughter, she is also my Mothers sisters daughter. My parents brother and sister are married to each other.
In the Quran you may read:
O Prophet, indeed We have made lawful to you your wives to whom you have given their due compensation and those your right hand possesses from what Allah has returned to you [of captives] and the daughters of your paternal uncles and the daughters of your paternal aunts and the daughters of your maternal uncles and the daughters of your maternal aunts who emigrated with you and a believing woman if she gives herself to the Prophet [and] if the Prophet wishes to marry her, [this is] only for you, excluding the [other] believers. ... (33:50)
So basically from a halal-haram or shari'a point of view such a marriage is halal, but one should or is strongly invited to do medical check for hereditary disease etc. before marrying to stay on the safe site based on this fatwa (in Arabic from the General authority of Islamic affairs and endowments of the U.A.E.. However this fatwa islamqa#139517 denies that marrying a close relative per se is a reason for hereditary diseases of the offspring.