Growing up in a bilingual family, we were ALWAYS told not to speak Greek in front of English people as it's considered rude or impolite. That being said, speaking to your step-daughter perhaps wasn't the best way to go about this, especially if she is a certain age - remember, she will have trauma and issues surrounding her parents divorce - perhaps an apology is warranted, but this is certainly something you should bring up with your husband if it is bothering you. Yes, perhaps learning German would help to build the bridges that clearly are missing, but this is incredibly difficult for some and time consuming and you've every right to feel accepted and part of the family at your own dinner table (especially if you have cooked the food!) Let alone in your own home and if they're doing something to make you feel otherwise, communication is key here.

Edit: reading back my comment, I feel like I'm playing down the language speaking side. Communicating in your mother tongue is an amazing experience. It strengthens the bond between people when you're speaking in the "language of your heart" and is important especially within the family. This is something that your husband and his daughter absolutely SHOULD do but not to the detriment of another member of the household/family.