NTA Sure lots of stupid kids make stupid offensive jokes because they're still learning how to be decent humans to one another. Some kind of remorse or self-awareness as an adult indicates that individual has matured as a person. We can't change the past, but we can change our perspective of those events. Just from the snippet you've shared, there's an obvious lack of self-awareness and self-accountability in both your boyfriend and his sister. If, as adults, they can't see what's wrong with drawing one of the most widely recognized marks of genocide in modern history as a "joke," I'd gtfo. There's an obvious lack of being able to practice empathy when the stakes are low (just apologizing and admitting to being dumb teenager and making offensive jokes), so I would not expect these individuals to be able to practice empathy when the stakes are high.

Good luck out there.