Kelvavik

Here's an update: I told her I would sign the form if she went to counseling with me. Initially she was furious and said she wouldn't go to counseling for anything, but ultimately she capitulated and we went to both a counselor at her church and one at our medical clinic. Through this process, it became clear to me that her divorce threats are empty threats and that she really wants a loving & supportive family. It's just that given her maturity level and that she thinks I'm afraid of divorce, she uses that any time she's angry such that it now almost means nothing. The key to identifying if one is serious with a threat - as opposed to just being upset - is to ask for the threat in writing and/or ask her friends if she's serious. She's made other threats (not divorce threats) which I haven't complied with and she never followed through with the threat.
She's also honoring my request that she find a place for her mother to live. (our place is too small to accommodate an additional adult, along with the fact that I often need to work from home). This was a condition I placed upon her in exchange for me signing the form.
There are some who have posted in here who say the mere mention of the word "divorce" is enough for me to run and/or not sign the i-864a. For those who fall into this category, I want for my marriage to work and value the positives my wife brings to our family & household. There are far more positives than an obviously empty threat which periodically resurfaces. If there weren't any positives to our marriage, I would have done precisely what you suggested a long time ago. But I'm guessing that you've set your bar at a different level than I have such that in your value system, one simple misdeed means the marriage should be terminated.
There are some who have posted in here who say they don't know the relationship well enough to draw a judgement on what it means that my wife threatens divorce.
There are some who have stepped back from the fact that a threat was made and offered information on what signing the i-864a means. Thanks for your feedback.