What if my Dad refuses to physically leave the house to move into assisted living?

It is a difficult decision that we all must make. I am sorry that you have no options. Fortunately for me, I lived about 45 min. Away from my mom and saw her on a regular basis. When she started to talk about assisted living, we had the option to move her into our home. I have just retired and spending it with her is a dream come true. I am blessed that my husband is who he is and respects women and loves my mother and appreciates her as well. We are looking forward to any time we can spend together. Right now she is in a rehabilitation facility recuperating from pneumonia, but hopefully she will be coming home this week. She only has a small room here, but we are adding a large suite for her which will be done within a month or two.

Scarsdale: good questions. A few things...
1. Right now, both my dad and stepmother require some help around the house. My dad does not cook, nor would he bother doing laundry or cleaning, and my stepmother is battling cancer (although, all things considered, in pretty good shape).
2. The nearest close relative (my stepbrother) is five hours away in case something goes wrong.
3. As my dad cannot drive, he feels trapped in the house, and thus a bit alone/isolated. Assisted living, I think, will restore his sociability.
4. Finally, there is a financial consideration. My dad is running out of money, and we cannot long term afford in-home care--but he does have a long term care insurance policy he took out on his own about 15yrs ago.

Why does he need to leave his home? What are the exact issues that makes you think he needs to move into assisted living? My mom is 89 and lived on her own until recently because we wanted to be together now that I can spend time with her. Moving away from your home is quite dramatic and emotionally traumatic. You no longer have control of your environment and it is a sign that your life. Is coming to an end, especially for a man who has been a provider all his life. His things are ther. He is comfortable there. His memories are wrapped around every pillow. Additionally he will have to go through his things to determine what he can keep, what gets given away and what gets thrown away. This is a lifetime of things that represent who he is. Put yourself I. His position before you assume he has to move into assisted living.

Eddihaskell:
"Is your father hurting himself through poor judgement about his health or finances, or living alone?" He pretty much cannot live alone; there is some concern over finances (he has an overdue income tax bill), but much of it has been automated. He was able to live on his own for three months while my stepmom was selling her house, with the help of Visiting Angels--but we cannot afford Visiting Angels as a longterm solution.
"If you already have an excellent and trusting relationship with his wife, then speak with her." I have a pretty good relationship with her; however, she is 86 and battling cancer (although doing quite well)
"I need to bring up the status of your fathers estate if there is one." They signed a pre-nup, and there's not much of an estate. He's got about $50K in cash left and the house is in a trust in my and my brother's names.

Guardian with a court order is the only way to go. See a lawyer asap, the process takes months to complete.

Is your father hurting himself through poor judgement about his health or finances, or living alone? It sounds like he may be.90 years old is an age where judgement impairment can affect a large percentage of the elderly -- and it does not have to be alzheimers.
If you feel he is, you need to consider the possibility of having him declared incompetent and having a court appoint a guardian to make important decisions for him. I believe (and I am not sure) that his wife will automatically move into this role. She will then be the one for you to work with to benefit your father assuming she is capable of making these decisions. I am not sure what your local law has to say about this (continue below about a lawyer). If you already have an excellent and trusting relationship with his wife, then speak with her.
I need to bring up the status of your fathers estate if there is one. Will his wife automatically inherit his assets if he predeceases her? The reason why I ask is that this is a primary cause of dissention in similar situations. If his wife is ever declared in control of his finances, or given the chance to change his will, you need to take this into account.
In summary, you need to confidentially speak (and do this initially without his step wife knowing about it) with a trusted family law attorney. Use one only through strong recommendations if you can. You need some legal advice and counseling, or just a clear view of the situation that an experienced family lawyer can offer.
The reason why I say this is that I hired a lawyer who gave me very poor advice a few years ago, and if I would have checked with the the State and Local Bar associations beforehand, and have done an online search for complaints about this attorney, I never would have hired him. Do your homework first before you see legal advice.
But it sounds to me like you should do this step to know what your rights are even if you never will bring an attorney into the situation.

Depression is treatable. Even the early signs of dementia are treatable (though not curable). Trying to encourage him to see appropriate doctors would be appropriate.

If he refuses to leave the house on the the day of the move, he is going to have to face the consequences of living separate from his wife and of having to pay for two residences. The consequences will be more uncomfortable if his family does not enable him by providing lots of help. Perhaps the consequences will eventually persuade him in join his wife. Of perhaps it will take a fall or some other emergency to force the issue.

Unfortunately, unless you are his legal guardian you cannot force him to move out of his house.

Unfortunately our parent(s) will stick like glue to their current home, not wanting to move anywhere, no matter how wonderful the new place might be.

My parents [mid-90's] are like that, and there isn't anything I can do about it. Since their mind is still intact, it is still their choice. So I am trying to learn to keep arm's length from helping them too much. Whew, not easy as a lot of guilt gets thrown my way. It will have to take something very important, like not being able to get upstairs to the bedroom, or a fall down the stairs to finally shake some sense into them.

Please get him to a geriatric psychiatrist for his depression! That's the first step. It's not a choice. You make the appointment, you tell him you're going to lunch and stop in on the way back ( or on the way there).