تقدیر یا توکل ؟
The best answerr to this was give in this hadith Anas bin Malik narrated that a man said: “O Messenger of Allah! Shall I tie it and rely(upon Allah), or leave it loose and rely(upon Allah)?” He said: “Tie it and rely(upon Allah). https://sunnah.com/tirmidhi:2517 So you need to take all measures then trust in God for better results. P.s The probability of disability in cousin marriage is 4-6℅ while in unrelates parents 2-3%. It about carrying recessive genes not cousin or not cousin. So getting gene info is necessary to prevent heriditary issues in child.