Well, I didn’t tell them for a very long time. They knew I was interested in Judaism, but I didn’t tell them that I had started to practice some things. We had plenty of other problems with each other (I was 16!). As they and all of my siblings are not religious at all, already that I was involved at the church was strange. They told me repeatedly that religion is for those who are not able to make their own rules. They knew I learned Hebrew, but I had taught myself Spanish and a bit of Russian before, so they just assumed I was into languages (which I am). They thought Judaism was a phase, one of these things people find interesting as teenagers and then drop it as they become adults.
Probably the first sign that I was more serious about Judaism than they realized was when I went to Israel for six months after finishing high school. But possibly they still thought it was a phase or that I just wanted to annoy them. Anyway, I went and I think the distance and the fact that I was so determined to go really helped both sides to re-evaluate their positions and expectations.
When I came back from Israel, I moved away for my studies and we started to get along better in general. And after a few years they even asked me if I was involved in the Jewish community and I said yes. It became more normal for them that I am sort-of-Jewish. They now see that I haven’t become a crazy fanatic. And now, more than ten years after I started this process, I can talk to them about my frustration with the conversion process, about a nice shiur I had, about some mitzvot [commandments] or about the ultraorthodox in Israel without any stupid comments. But I still wouldn’t tell them all that I do and I make plenty of compromises when I am at their house.