My dad is cheating on my mom again and I am the only one who knows
I am 19(f) and my dad has cheated on my mom on two separate times before, first when I was only a baby, where he had a son outside of marriage, (my mom knows and has forgiven him, neither me or my older sister knew until last year) and second, with another woman who convinced him to leave the house and "divorce" my mother (he never did because that would mean paying maintenance). When this happened he would often thretean my mother with divorce, saying he would leave her with nothing. He later broke up with her and apologized to all of us, came back to live with us and things have been fine until now, where I saw him texting this same woman making plans and telling her he loves her, and now I don't know what to do.
For context, my mom stopped working when he married my father, she doesn't have anything in her name and my sister lives in another country, she doesnt have a house or citizenship there yet, and may never will. Her mental health was also extremely bad, and even thought about killing herself (she never tried though), she has been recently going to therapy and is much better, but I know that telling her about this would probably destroy her again. I also don't was to tell my sister because I know she would take this way harder than me and wouldn't be able to keep it a secret. I don't know if I should even confront my father, since I extremely doubt he will stop seeing her just because I tell him, and I don't even think he will ever change.
I really don't know what to do, please give advice