Judging my sister

I am judging my sister for being selfish and ungrateful for the things my parents have done for her which is way more than they did for me or my other sister. I want to stop judging her but I truly believe she is selfish. The coach says “All relationship work is really work on our relationship with ourselves. You’re getting a chance to see your own thinking and to be curious about that. ” How do I get to non judgment on this? How does this reflect on my relationship with myself?