It's actually simpler than it sounds. All you gotta do is count the number of steps up and down between direct ancestors/descendants. So for example, 2 up (your grandparents) then 1 down (your grandparents' children) are your aunts/uncles.
2 up 2 down is first cousins
3 up 3 down is second cousins
4 up 4 down is third cousins, and so on
How "removed" works is to look at the smaller number between the up and down, and then take the difference between them. So 5 up and 8 down would be the same as 8 up and 5 down. Between 5 and 8, the smaller number is 5 (fourth cousins) and the difference is 3 (three times removed). So both people would be fourth cousins three times removed to each other. Anything with the smaller number being at least 2 is some kind of cousin.
All that said, you can go look this up yourself by googling consanguinity. In culture, law, and genetics, 7 total steps (such as 3 up 4 down which is second cousins once removed) is far enough to be considered not related anymore. I doubt MC and his stepsister are that far apart, though.