should my mom and sis go together for the visa interview?

Hello Everyone,
Happy New year!
I have a quick question:
I am planning to invite my mother and sister to US for my graduation
ceremony in May.
But I am not sure whether it is a good idea to have them apply for the visa
at the same time. Since my mom only has two children, if my sister is also
applying, do you guys think that will affect the chance of my mom's visa
being approved?
btw, my sister visited US twice before for biz. she has very strong tie with
China because of her family and work.
What do you guys think I should do?
should I arrange them go together? if so, do I need to fill out one form or
two forms, each form for each person?
Thanks for any suggestions!
have a good day!