You may know a definition of moral relativism, but you don’t know how to apply it.
You can throw a hissy fit, and not go to your grandmother’s (which is likely to hurt your grandmother), and you can brew up a big fight with your mother - which is if course going to really clear the lines of communication between the two of you, won’t it.
You have already expressed your opinion to your mother, and if I read you correctly, you have engaged in moral relativism, as you want her sexual activity with her boyfriend to be hidden from your grandmother.
Her sexual activity with her boyfriend is wrong whether it takes place at your grandmothers or somewhere else. According to your post, however, that is not what you said.
You may well have told her it was absolutely wrong, but I can only go by what you related, which is a qualified “don’t do it at Grandma’s”. and that qualification is likewise moral relativism by your definition.
I don’t recall seeing your age noted anywhere. If you are an adult, you have said your piece, and need to leave it there - your mother got it, even if she is choosing to not change her behavior. You don’t need to keep at it as if she were not capable of understanding what you said.
And if you are a youth, the same applies. You said your piece. you now need to respect your mother, whether she is sinning or not. Tend to your own sins. She is an adult; she knows what the Church teaches (as is obvious by her response) and has rejected your request.