How do I react to my MIL when she blames her problems on me?

You did the right thing. Your MIL doesn't like the way you help her, so no reason to keep doing it. Let your husband help his own mom. She might be nicer to him than to you.

Stepping out of the caregiver role is probably a good move at this time. It doesn't have to be forever, but while she is in this phase of the dementia it might reduce tensions.

What you are experiencing is quite "normal" in the dementia aspects of Parkinson's disease, and other dementia. The person gets paranoid. Nothing in their world is going right. It has to be someone's fault, and often they pick the person who is around the most. (Sometimes they blame a neighbor or a "bad guy.")

My husband's paranoid period lasted several months -- the hardest months for me of the entire 10 years! I was stealing his money. I stole his car. I was holding him against his wishes. He told this to the neighbors. He tried to tell it to the sheriff, but fortunately he couldn't dial the phone well enough.

If you can detach yourself some, do it!

Here are some tips for when you do interact with her:
1) Even though she is accusing you, it really isn't personal. She was nice to you a few months ago. She still likes you. She just needs someone to blame. You're it, for now at least.
2) Arguing and using logic is not effective in this situation. Save your breath.
3) Go along as best you can, and change the subject. "Well, I sure didn't deliberately hide the bills, but I guess I might have misplaced them. How about if I look for them right after we have some ice cream?" "Yes, this cell phone really is fancy, isn't it? I got a lot of features so I won't have to change it so soon. Did you ever think we'd be able to carry our telephones with us?"

Has the medical staff talked to you and your husband about dementia symptoms within Parkinson's? That might be helpful if you could arrange it. It also might help to attend caregiver meetings. The more you know about what is "normal" in this situation, the easier it should be to not take it personally. Still not easy! But a little easier.

I was also going to mention Parkinson's dementia. You can google that term and read about it. My friend's sister-in-law has Parkinson's and now the dementia is showing itself. Hers is more memory related, but there are different variations. Stepping back sounds like a good idea. But knowing that it's the dementia talking and not your MIL should be of some comfort. You can also watch Teepa Snow videos on Youtube about dementia. They're very helpful. I'd be more upset that your husband is being so non-supportive. He needs to do some reading and learning.

I didn't see where Dementia was mentioned by OP. Just assuming that because she has Parkinson's she had Dementia? If this is a new problem, then have MIL evaluated to see if she does now have Dementia. If so, there are meds that will help with her paranoia. There are other things that could effect her thinking. UTI, low potassium, sugar etc. Good thing to back off. No one needs this abuse. Let her son take over and then he'll see or she rather have him. But anytime there is a personality change there is a problem. Is her cancer being treated. Melanoma travels. It could have gone to her brain.

Don’t you know Perlls, Happiness is knowing who to blame.
Hugs

This is jewels, I want to thank all of you for your valuable feedback, I felt so alone, you're all angels, thank you so much, I feel better now that I have decided to let my husband take care of his mom for now. God bless you all.

Stepping away is the best idea. My MIL hates me (she has told me often enough, I truly believe her) BUT, she has hated me for 40 years, so this is old news.

Yep, sons can be clueless. My hubby wouldn't "defend" me if she was throwing gas and matches at me. He simply does not DO confrontation and "talking". She is now almost 88 and lives alone. Drives very short distances, but is pretty much a hermit, which is what she wants.

LET HUBBY do all the things. I don't do anything for my MIL. She'd be very upset if I even tried.

Smarter to step away and have some peace than hear how awful you are all the time. I wish I had bowed out of her life completely many, many years ago.

Dad was so very hard on me. Accusations from taking his money to taking his life from him, all while I was trying to take care of him. I remember my sister visiting (didn't happen too often), and he was so kind to her. But all of his complaints, agitation and "things to do" were saved for me. We argued early on, when I didn't really understand that it was his dementia causing the accusations. Later, toward the end, he was so grateful that I was there. And I was able to comfort him just by being there, not doing anything. Parkinson's is a terrible disease both for the patients and the care-takers.