How do I tell my younger sister to stop correcting my German?

My sister (from dads second relationship) was born and raised in Germany, she is 12 years old and I see her every year when I visit dad in the summer. Being German, but raised in English speaking countries, my German is at a high enough level to communicate on a day to day basis, but it's not perfect. I obviously have grammar trouble when speaking German. My younger sister keeps interrupting and correcting my German if I mispronounce one single word or use a wrong case. I found it cute at first, but she does it every single time I open my mouth and speak German. I can not give my dad a full sentence without her rudely interrupting me. She is learning English in school and to try to shut her up I tell her that her English is not great either. She just tells me to "shut up" or "well I am an exception, I have not been speaking English my whole life."

How do I really tell her to just stop as it is not funny anymore? Thank you