In 1565 she married her young cousin Henry, Lord Darnley, much to the chagrin of Elizabeth (Darnley was a grandson of Margaret Tudor and thus also had a claim to the English throne).
In the old days, marrying a family member was a normal thing to do. 
mnli hypothesis: In the old days, marrying a family member was a normal thing to do.  premise: In 1565 she married her young cousin Henry, Lord Darnley, much to the chagrin of Elizabeth (Darnley was a grandson of Margaret Tudor and thus also had a claim to the English throne).