MIL named a medical POA, but not a financial POA, and I don't know what else hasn't been done. Any advice?

I would have thought the medical and financial poas would have been done together? Yes she has done a will, but I am unaware of whether she has a dnr or anything of that nature (whatever that is called - a directive?) Yes, she has notes, all over the place about her funeral.

Has she made a living will? Has she made a will? Has she said or written down any wishes about her funeral?

Weird that she did a Medical and not a Financial. They usually go hand in hand.

Weird that she did a Medical and not a Financial. They usually go hand in hand.

"Is a financial POA a necessary thing?"

Yes it is!

Without it, you will not have the legal authority to spend her money on her care, deposit any checks that come to her, manage her investments if she has any or do her tax returns once she becomes mentally incompetent and thus cannot conduct her own business in a business like manner. Like Yoda said in Star Wars, "Do or No Do, There is no try."

Toadhall, most definitely Lostinthemix's husband should be involved, but Lostinthemix is also heavily invested due to MIL living in her home for past 8 months and is probably providing much, maybe most, of MIL's care. Lostinthemix, I neglected to see in your profile that your husband has "siblings who don't seem to really shive a git." My experience is that such siblings are often the ones who start shiving a git when finances become involved, so I'm amending my prior advice to align with Toadhall's, i.e. make an appointment with a good elder-law attorney ASAP. Kudos to you, Lostinthemix, for agreeing to help MIL with her dementia under trying conditions. Good luck and best wishes to you and your husband.

Since this is a mother-in-law shouldn't the child of this person be dealing with this? The best thing is to get them, parent and adult child, to an elder law attorney. The sooner the better--do it yesterday.

Lostinthemix, yes, in order to avoid guardianship/conservatorship, a financial durable power of attorney (DPOA) will eventually almost certainly be needed. A person can still legally sign and have notarized a DPOA even after an Alzheimer's dementia diagnosis, but only if that person is reasonably aware of what the DPOA document is and does. "Reasonably aware" is a somewhat subjective determination and if there are any family members who might cause trouble later, then it will be best to involve a good elder-law attorney who can advise whether, at this point, a financial DPOA is still an option or if, instead, guardianship/conservatorship is necessary. Here's an AgingCare.com POA article:
https://www.agingcare.com/articles/difference-between-POA-durable-power-of-attorney-living-will-140435.htm

Also, if you do an on-line search for DPOA, guardianship and conservatorship specific to your state, it'll probably bring up useful and easy to understand information from your state's court and/or bar association.