I'm sure that they have encountered this problem more than once over the years. They have better enforcement powers than you or I will ever have.
You were supposed to have received Copy B of the IRS Form 1098 by February 2nd. She's a month overdue, unless she asked for an extension. There are probably criminal and civil penalties if she never files the form at all.
If you get near April 15th and still haven't received a resolution to your problem, you can always file a tax return and estimate what you think your allowable deduction would be along with an explanatory letter indicating that the previous mortgage holder failed to provide you with the what you needed.
You can also obtain a copy of the Form 1098 and send it to her with along with a note explaining the situation. Ideally, you can do both. You shouldn't threaten her with going to the I.R.S., since that could be construed as blackmail.
Here are some publications that you can include. http://www.irs.gov/publications/p936/ar02.html#d0e1705
If you want a copy of the 1098, go this this link and download it. http://www.wwwebtax.com/irs/irs_forms_list.htm Apparently the service is hinky about accepting downloaded copies, so you may have to obtain a hard copy from the IRS.
I hope that this helps you a bit.
Ultimately, you have the better hand, since she is the one violating the law and will get in trouble if she fails to do what she is supposed to do.
Good luck!