Mom is showing signs of dementia but refuses to seek medical care. It's impacted her husbands care and their finances, what can I do?

Do your parents have a Trust? Do they have an Attorney? If they have a Trust find out who they have appointed as POA when "the time comes." These are normally set up prior to the need for them as a person who is mentally ill is unable to appoint a person as POA. I realized my Mom was having a problem and immediately took her to the attorney to have POA assigned to me as I was her caregiver. She was not sure she wanted to do it, but I explained to her that it was not for use right now, but for use in the future when or if it became necessary. This is when stretching the truth came into play. Mom's attorney spoke to her alone and then had me come back into the room as he wanted to make sure she was okay and not doing this under duress. If you do not do this NOW you will go through hell later without one as you then have to procure guardianship which is long, involved and very expensive! In the meantime your parents could go broke!

Try to get her to a doctor to find out if it's dementia, ALZ, an infection, etc. But, if she won't go, that's her decision. And, if it is dementia, there's not a whole lot of 'treatment' that's going to help. Even with treatment, it's a degenerative condition.
The priority is rescuing their finances before it's too late (I speak from experience!). Plead, bully...do whatever it takes to get a look at their expenses and income and debt. Get your siblings on board and approach mom as a team. Whatever tactics work (reverse psychology, bribes, etc...), use them to help get their money in order. Every penny they squander means fewer choices for them in terms of their care and their future quality of life.

Have you explored your options from a legal standpoint? Seems like that would be the first step. If you can't afford a private attorney, try the local legal aid organization. Taking advantage of community resources for the elderly will take some of the load off your shoulders and keep you informed at the same time.