我這個週末要照顧生病的妹妹，所以無法赴約該怎麼說呢？

謝謝~

I cannot go to the appointment(職場正式會面／約診)/get-together（朋友聚會）/date（情侶約會） this weedend because I have to attend to my sister who gets sick.

I can't make it this weekend because I have to take care of my sick sister.

「無法赴約」也能說 I can't make the appointment.