Performing qasar salaah when staying in a place for less than 14 days

Q: A person went to a city intending that he will stay there for 14 days and was performing qasar salaah alone until he noticed his wife and mother were performing full salaah. He told them that their stay in the city was 14 days and they should perform qasar salaah.

1. Is it correct?

2. I also heard that the intention of a man is counted if he is with his family. Is it correct?

Children born with disabilities due to marrying in the family

Q: In Islam, is cousin marriage allowed?

Medical science says that cousin marriage is wrong because it causes disabled children. Yes, there are many cases where children are completely fine but there are many cases where children are disabled because of cousin marriage.

My Conclusion: cousin marriage is risky.

Please advise me whether we should do cousin marriage or not? If your answer is yes then how can we overcome this risk?