Hey, please suggest me what should I do?

This shit happened to me and I have no idea what should I do now.
Me and my long term girlfriend, we live together. Recently my girlfriend's sister came to our home, for few days. But because of lockdown, she is also stuck with us.
Not a problem, we can still manage everything. So as usual, me and my girlfriend have sex quite a lot, almost once or twice daily, after all what else is left to do during this free and boring time.
Her sister usually messes around, more like flirting. I reply in same way, but nothing romantic, just for messing around.
I felt something strange around her sister since long time. But last weekend, after doing it with my girlfriend, I went down to watch TV, while my girlfriend was sleeping.
Her sister also joined me watching. We were talking and she was sharing how sad she is because of her sexual life.
While talking she did what I never expected. She kissed me, and wanted to have sex with me. I was literally scared that time, she was sitting with me topless and trying to have sex with me. I just ran back to my girlfriend and slept with her.
Her sister wants me to cheat on my girlfriend, to do it with her. I seriously don't wanna cheat on my girlfriend. What should I do now, if I tell my girlfriend, she will be emotionally broken. I don't wanna hurt her.
Please suggest me something..
Hey, please suggest me what should I do?
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