Why didn't you attend your class yesterday?
B1:I am sorry. My sister had a class where I needed to help her.so I was absent from my own class.
B2:I am sorry. My sister had a class with which I needed help her .so I was absent.
B3:I am sorry. I helped my sister with her class,which was teached in the same time as my own class.so
B4.I am sorry. I was absent for helping my sister with her class which had the same schedule as my own class.
I just want to know which expression is more natural.
Not only the expression I gave above, but the better one you think,which has the same meaning.