I don't think Stacey is the right age to be the MC's sister (if he even has one).
Here is my thinking -
MC, Jaime and Shauna leave school and go to college at age 18, so the existing basketball players and some of the cheerleaders must be aged 19 or 20 (as it is stated that they were there in previous year/s). Their age is irrelevant as they are all 18 or older.
Elizabeth is clearly older than Jaime (yes, it is possible for two sisters to be born 10 or 11 months apart and be in the same school year, but unlikely), but that could be due to her taking a gap year or retaking a year for some reason, either way she is over 18.
MC's mother left when he was a toddler - i.e. at least 18 - 24 month old. If he had a sister at that time Azel would have known as she knew his parents (well enough for his father to appoint her the MC's legal guardian if anything happened to him (police do not turn up with random women when a child's parent dies)).
Therefore if MC has a sister she would have to have been born at least a year, perhaps two, after MC - and would not be old enough to either attend college (or star in a porn game).
I may be overthinking this (I often do), but I'm inclined to think that Stacey is not "sister material", but I reserve the right to change my mind at any time.