good advice from hannah,softly softly approach will work imo.
i had a terrier bitch who was the same(the mrs loved her and so kept her,she was tiny and very submissive/sensitive) i never worked or tried to train her,the girlfriend did everything. i was living on a farm at the time and noticed her ratting with the others,fast as hell and really good. next time i went checking the barns for polecats and martins i took her with me,she could fit every where and bolted 2 martins alone.next time out she killed a polecat( yes she stank to high heaven).she was around 12 months by now.
i took her on