"but there seems to be an element of moral panic to you post which I think is unnecessary." Let me guess you're a cannabis user yourself either now or in the past.
"and many people these days don't see weed as any "worse" than drinking." And they're wrong. It is still an illegal drug that is unregulated and can have serious effects on the user.
"Studies have found no detrimental effects to health unless it's 'hash' which is the cheaper alternative which has been linked to psychosis after long term use." Where you getting that from? First time use can and does lead to psychosis and other MH issues.
There's also the issue of smoking in the house generally (weed or tobacco) and secondary smoke effects on the child. That would annoy me too.
I'd be thinking it's worth your dh speaking to his sister to see how she really feels about it, if she's doing the same, how often etc.
Personally in your position I'd not be letting either of them care for any child of mine and I'd be keeping an eye on sil & the child for their own wellbeing.
The point is it is still illegal (though you'd be forgiven for thinking otherwise given the lax approach even from police now which I don't agree with).
As pp said driving is also a concern, drug driving especially under influence of cannabis is on the rise and it's just as dangerous and irresponsible as drink driving.
"Adult weed smokers will defend and miminise like crazy" yep see that on here SO much
"One of the chosen illegal drugs that MN has collectively decided is fine / socially acceptable / "it's only" / etc" exactly!
"Mumsnet always amazes me with its double standards, Canabis is okay in countries where it is legal and regulated. In the UK it is not OK, it’s and illegal unregulated drug, purchasing it her is dont in shady back alley deals, it is putting money in the pockets of dealers who dont just deal in canabis but all manner of illegal life destroying drugs and the kind of criminals that go around stabbing people in turf wars. But hey that’s okay because after all it’s just weed." Totally agree!
"I think the mental health repercussions tend to be caused by people smoking it a lot/ mixed in with other drugs" not always true, plus as its unregulated users don't necessarily know if there's other substances included.
"Or grown at home, cutting out the dealer ?" Still illegal and while it cuts out the unregulated factor, also means the grower could be done for possession, production even supplying if they're even giving it to anyone else.
"It also is far less likely to lead to violent or unpleasant behaviour." Not necessarily true given the MH implications.
All this "no worse than alcohol" as the child of an alcoholic and having witnessed first hand the effects of cannabis use on relatives and their families, both awful drugs that wreck families & their users health. People are more aware of potential issues with over indulgence of alcohol, cannabis has similar effects with lesser use and changes brain chemistry quicker leading to addiction.
Speaking to Dh's parents would likely be pointless and just cause arguments and achieve nothing sadly.