yes she did understood there was something more to it but lets not jump into the i want to bang him train! she saw him after 5 years even if she finds him attractive i doubt any sexual desire is there, only her concern to keep him close this time after loosing him for so long (that can lead to sex... but it does not automatically means sex)!
so her concern in regards to the girl is how much of an attraction is there and how likely is the danger of loosing him again if one of those sluts calls him back (from Erynn point of view they are sluts who might steal her little brother)!
and as
LSC82 stated her concern is how much he is liking to be back since that is a determining factor if he will stick around or leave again!
basically we can speculate that the MC is the only positive male figure in her life!
If we read this from 13 years of knowing each other she probably has emotional claims to the MC and he was her little brother, anything else is perv. thinking! does not mean it can not grow from there!