It sounds like things are really difficult for you right now and I'm so glad that you've reached out for some support and assistance. You have a lot going on and it's so great you have recognised that you're struggling and need help. It's probably not surprising that things feel out of control when your family is struggling to cope with the death of a loved one, who was also a 24/7 carer. There is so much adjustment that will need to occur, both emotionally and practically, to reconcile the loss of your FIL.
You have a lot of insight to recognise that the things your MIL has been saying are a reflection on her, and her (mental) health, rather than on you. It sounds like you are doing everything you can to support her and your husband. Her reactions seem spiteful and are probably really hurtful for you given everything you are doing, but it sounds like they might be coming from the pain and grief she is experiencing.
Are you and your partner able to openly discuss how you are feeling and how this is affecting you? You are in this situation together. He will be trying to deal with the loss of his father and the care needs of his mother, as will you. Your perspectives and experiences might be different but if you are able to share what you are feeling it might foster some mutual coping strategies?
Or perhaps you (and your partner?) might benefit from some external support through a grief counsellor, or a Psychologist? You might want to consider seeing your GP and asking for a referral through a Mental Health Care Plan to a Psychologist. You would then have access to six (possibly more) sessions with a Medicare-rebated Psychologist.
I hope you are able to get the support you need. It sounds like a really difficult time for you right now,