First of all, Mendel is an Austrian imperial biologist, born in Brno, a botanist and geneticist in Moravia, Silesia, Austria. As the founder of modern genetics, he carried out the famous pea hybridization experiment and put forward the law of free combination and separation, which made great contributions to the development of genetics.
So there is a factual error in this question. Mendel has no close relatives to marry.