My sister was ovulating (says she was based off cervical mucas, nothing else) and had sex the day after... she is trying to get pregnant but doesn't know if the day after makes a difference, I'm not sure either... because I'm not sure about how the cervical mucas plays a part in the ovulation process...

When ovulation is close, cervical mucus will change to allow sperm to survive for a few days, otherwise they die off fairly quickly. For some women changes in cervical mucus can be quite reliable, for others it may not be. Often changes will be noticed a few days prior to ovulation... but it can be different for different women. Might be noticeable the day of ovulation or not at all... which doesn't mean ovulation didn't occur. (just as having a change in cervical mucus does not guarantee that ovulation took place)
Her best bet, after paying attention to possible changes for a few cycles to pick up any pattern, is to start trying as soon as she sees a change (often referred to as being like egg whites) and for the next several days afterwards.
Even with good timing, it can take quite a few months before achieving success. For some couples a year or so can be normal.
Good luck to her!