The short answer is no, the IRS cannot seize your fiancee's social security, or any of her other assets for that matter - so long as she doesn't give you an interest in them - to satisfy your tax debts; those tax debts are your debts, not hers, and cannot be collected from her.
Beyond that, however, did you file tax returns for the years the IRS says you owe taxes for? If you did, you may have some options to deal with the tax debts. You should seriously consider consulting with competent local tax counsel to get an idea of what your actual options may be. A consultation won't give you a complete answer, but it also won't commit you to having to pay a lot of legal fees just because you consulted with someone.
My answer does not constitute legal advice and may not be relied upon by anyone for any purpose and does not constitute an attorney/client relationship or an offer to form such a relationship. This disclaimer is intended to be fully compliant with the requirements of Treasury Department Circular 230 and the terms thereof are fully incorporated by reference. If you wish to consult with me please contact me at dwatchley@newyorktaxcounsel or visit my website at www.newyorktaxcounsel.com
As Attorney Atchley pointed out, it is important that once you are married (and even before really) you do not co-mingle money in joint bank accounts or other assets, once the monies are mingled the IRS might come after them - this would then require you to put forth the effort and evidence to show which monies / assets are yours alone versus which belong solely to your spouse versus which are marital property.
Disclaimer Information on this site is provided by Brian Scott Wayson as general information, not legal advice, and use of this information does not establish an attorney-client relationship. If you have questions about your specific situation, please call an attorney.