In cultures where marriage decisions involve a lot of people, it is the parents who must consider these things carefully. To do otherwise would be irresponsible.
I suggest you go to a community leader or spiritual leader that knows both families and ask for a candid conversation. Then contact a genetic specialist, or even a student who studies genetics and ask the right questions. You need to gather more real information discreetly to make a wise decision
I am a bit confused as to your question. First off who is "we" and why would these things about the brother and sister be relevant? Second, the word "retarded" is politically incorrect and should not be used. As a mother to an autistic son, I found this question to be a bit close minded and particularly offensive. These things that you mentioned should not matter.
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