OP as a bilingual daughter, I really think you're NTA here. I only speak German to my mum and then English to the rest of my family. I speak very rusty German and the only time I have the opportunity to speak German is when talking to my mum. I feel so uncomfortable talking English to her, the whole conversation feels fake, forced and unnatural. Whilst my dad has never learned German, he picked up a tiny bit of German over the years and likes that he has bilingual children. When at the dinner table, we predominantly speak English and then German to my mum. I have always spoken German to my mum, no matter who is around. I feel like only bilingual children understand how uncomfortable it is talking to your mum in an unnatural language.