assalam walakum!!

I have a quiery, my older sister is 20 years old and wants to apply to a university in another city which would mean her living there,she says it offers her the exact course she wants to study,

in islam i know this to not be permissible as a mehram will not accompany her,

my sister has a very fiery temperament and never really listens to anyone, my mother and father also do not know how to react and are very worried what is their duty on this matter,

how can i advise her diplomatically without angering her

shuqran in advance.
sister in islam.