My wife is an A cup.. and she's got more than a 3" difference in her two measurements.
If she's outgrown her current bra....she really does need to talk to her mom....panties don't just grow on trees...bras while a tad more complicated really aren't a lot different....meaning..."mom these panties are a bit too tight... I need a larger size"... and she will be talking to mom about her period and everything involved soon if she isn't already... and that would be a more sensitive subject.