You are still conflating.
I asked about reversing the roles of each sex here, & you responded with something about condoning drunks. I was asking you to consider why you think a mother can ban something a father wants to do - as a separate concept from the whole drinking issue. You took that as carte blanche to confuse the role reversal concept with condoning drunkeness.
So to make it clear - I have no issue with MiL being banned from sole care due to her drunkenness. What I was asking is why you feel that ban can be imposed, when one spouse says it's a problem, & the other spouse believes the opposite.
It’s very worrying that you think that her husband can just give his mother her children for the night.
It's very worrying that you think a father gets no say over his own children's care.
Removing the issue of MiL's drinking -what makes you believe that a woman gets 100% veto over her H's parenting preferences?
Of course he can’t!
You may think so, but the legal system disagrees.
Unless you can show me the legislation that backs up your view that a father isn't allowed to have his kids stay with his mother, just because his wife says no?
Of course she can stop them staying at her mother in laws.
How?
Why on Earth would her mother in law have that right?!
She doesn't. The childrens' FATHER has that right.
Seriously concerning that you think that way.
How so?
OP's best option is to convince her H of the harm that could befall the DC by MiL's excessive drinking, & get him to see that it's not normal - he probably believes it's harmless because it WAS his normal, if MiL drank a bottle a night when he was a kid.
If he refuses to come round to OP's view, she then needs to consult with professionals & hope he will listen to them. Starting with the GP to ask for the most appropriate service to advise her on her concerns would be helpful, so she can get expert opinion & back-up to help her H see her point of view.
Short of that, there's not a lot she can do. Threatening to LTB won't help - he'll likely farm the kids out to MiL on his watch anyway. Knowing she is right won't help - no matter how many PP fulminate that she has the 'right' to prevent her H taking his own kids to his own mother. It's him she needs to convince - not his mother.