Wow. Just....wow.
1. I'd keep the police out of this, if I were you. At best, it's going to cost you more than $1000 to get the rifle. At worst, you'll never see it again.
2. DO NOTHING ILLEGAL. This rifle, no matter the sentimental value, isn't worth the price of your integrity, or losing your right to RTKBA. Again--DO NOTHING ILLEGAL.
3. Pay her the $1000, or a cash offer. If she doesn't take your cash offer, be prepared to pay $1000, or walk away and forget about it. It's a lose/lose scenario if you do anything else.
4. I know the rifle has sentimental value to you, but if you don't move quick and she is a druggie, its days are numbered. Move quick, or be prepared to not have to worry about it.
5. An option that I've not seen offered yet: If your father and his sister are close, could you not have your father intercede on your behalf? Maybe have your father make a cash offer to his sister?
...Don't do anything that makes you out to be the bad guy in this. It isn't worth it. Don't listen to threads that advocate breaking the law, or trying to "trick" her--that's a road that you don't want to start traveling, my friend. Stay highroad! Be prepared to walk away if you can't do this right, and know that you did what you could. Personally, I see M1 Garands going for her price here in Southern California.
Stay highroad!
Respectfully, Sixtigers