Confused about whether I am becoming what I hate..?

First off, your sister-in-law is one of those judgemental snobs that you've hated in the past. Secondly, they say that the best form of revenge against people who don't believe in you is to get a higher education and become successful in you life. Third and finally, you are human and not perfect just as all humans aren't. People have said that the older you get the more judgemental you become. How true that is remains to be seen (at least for me in my life), but maybe you should change your approach to this. Instead of seeing a teenage mom and judging her - reflect back on your own experiences of being a teenage mom (diaper changing, midnight feedings, trying to study and take care of a baby etc.) and just say to yourself something like: "I remember those days, she's definitley got her work cut out for her there, lol!"