My gf's (25F) family won't accept me (27M) unless I convert to Judaism.

I'm going to apologize up front because this is going to be a mess.
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We have been dating for over five years. My girlfriend (Gf from here on) is a Conservative Jew and I don't have a religion. My parents were Christian as children, but raised me without religion. We still celebrate the Christian holidays, without all of the religious context, as a time for friends and family. Gf doesn't consider herself spiritually religious, but Judaism is a big part of the culture she was raised in and the traditions she follows. I love that about her, I like learning about and participating in the the traditions that have shaped her life and she does the same for me. Our relationship is great. We're both happily in love and haven't had any issues that we weren't able to talk through and come out better for in the end, except for one major problem, her family.
In the five years that we have been dating, I have been to Her house (while her family was there) once. When we first started dating I didn't mind too much. We are both very busy people. Gf works part time and is in her final year of law school and I work full time as a programmer while finishing a bachelor's degree.
The issue stems from her step-father, let's call him Dan and her mother, Jane. Dan comes from an Orthodox Jewish family. It was a major point of contention for Dan's family when he married Jane because she was a Reform Jew that converted from Christianity and they did not consider that conversion to be legitimate. Because of this, Dan