@User10932 i have filtered your replies only so apologies if I ask or comment as others before me.
You have mentioned that SIL’s behaviours can be fuelled by alcohol & this at present is not openly addressed. Her behaviour to you, your DH & your children has been unacceptable & more so as it took place in your family’s home/safe space. For now, leave her on the side of the plate as she does not interest you.
Which allows plenty other things to be unpacked that are within your control & may directly benefit you.
As you say, you are a child of an alcoholic who has recently been widowed. You may therefore have far more on your plate with her, as apart from your own loss of your StepF, you may no longer have the buffer that he provided.
If you are not looking at the impact on you as the adult child of an alcoholic, now may be a useful time for you to acknowledge this.
Al-Anon Family groups can be a great peer support for you to talk to as they have heard/seen/mopped up themselves after every imaginable event of being close to an alcoholic. You will be welcomed without judgement.
One of the interesting features of living openly & authentically around alcoholics is clear it is how many people deny or kid themselves about how alcohol drives the addict or substance dependent, & their friends/families to dodge addressing the impact of alcohol misuse on everyday life.
So @User10932, please help yourself by opening up your own stuff about your mum & you will so also become less of a target for SIL.
Imagine this: Because SIL does know that alcohol affects her behaviours & right now, her F&F enable & allow that by making excuses for her. But you do not do so for your mum, certainly far less than SIL is used to. By challenging your DM & in effect her alcohol driven behaviours in front of/within hearing of her, you scare SIL shitless as she fears that you will either come after her with your honesty or possibly enlighten your DH & she will have fewer places to hide her. Now her ranting makes more sense.
You are no way psycho, you are honest.
Your DH is not a mind reader & although he knew you were dealing with your DM, it is not for him to second guess how you may be. This is some of your unresolved stuff about being that hurt/frustrated child & dealing with that in the here & now. Don’t deflect that on to him.
Yes it is good that he supports & comforts you… as an adult.
But take care not to squeeze or guilt him into a role cast in the distant past.
@User10932 put yourself first & from there resolve what you can for the benefit of you, your DC & DH.
https://al-anonuk.org.uk/
https://al-anonuk.org.uk/getting-help/helpline/