Yup, they were cousins. The only reason Bloody Mary didn't execute Mary, Q of S, is that didn't live long enough. The day that Liz signed the order and Mary Q of S was executed, she kicked out the staff, shut herself up in her chambers, and wailed and bawled so loud it could be heard throughout the building. It's not surprising, though, that she never quite trusted her father's side of the family. Her first cousins Catherine and Henry were kept close, but didn't last like she did.
I’ve noticed that many monarchs and Lords died in their 40’s of ‘sudden illness’.
I know hygiene wasn’t very well practiced in those days and food spoilage was common as was plague, consumption (TB), etc, but I’m betting many were poisoned...................