In what way is OP 'part of the problem'? If the other party wishes to also pursue a romantic relationship - what's the problem?
A) It's okay to flirt with suspects/criminals if they are into you
B) It's never okay to flirt with suspects/criminals and you will lose your job if you do
?
If she is a suspect, then she would feel pressured to go along with the police's advances, because of the risk of retaliation for knocking back advances. So, B.
But for the victim of your sister's crime, she will want justice to be served, and will definitely go for B.