Could someone please tell me what the genetic problems are when blood relations marry?

Genetic problems don't always happen in intermarriages.
But the risk of developing an otherwise recessive genetic abnormality definitely increases, if it is in fact present in the family members.

"Ordinarily, two unrelated parents will have enough variation in their genes to ensure that their child has at least one normal copy of each gene. However, people with a common ancestor may each carry the same copy of an altered gene. This can make genetic disorders more likely to occur."

If you want to go ahead with an intermarriage, the safest option is to go in for Genetic counselling and testing.

Genetic counselling and testing. can any one explain in detail about this