Can your brother-in-law’s sister be your probation officer and not have it be a conflict of interest?
There is a difference of there being an appearance of impropriety and there being actual impropirety, meaning that if she treats you like every other person for whom she is the probation officer then no, buy on the outside looking in her actions may be subject to greater scrutiny becuase of the relationship.  I would not have her take the assignment for both your sakes.