I would strongly recommend you. Dh and dd go to nar-anon.
You ALL need to find your backbones and quit enabling them.
I'm also the dd in a similar set up, except I have gone Nc with the brother (in my case sister)
My parents are STILL bailing her out - she is in her 40's and a single mother, but she has NEVER done it all on her own, having copious amounts of free childcare (for weeks at a time), financial bailouts, emotional bailouts...
My parents both agree they should have nipped it in the bud DECADES ago.
My mum is genuinely worried she will end up on the street after they die once she's burned through her inheritance (which frankly she's already HAD at least 3 times over!)
Dad, brother and I and quite honestly the rest of the family and anyone who knows the situation have no such concerns.
She'll either finally step up (she's far from stupid or incapable of doing so) or she'll find some other mug to bail her out! She will NEVER go without - hell she won't even go without her luxuries! Including her drug of choice (literally)
You are NOT doing your son ANY favours preventing him from maturing and from facing the consequences of his actions especially his drug use, which as its weed can and does include psychosis and schizophrenia among other things.
You are also letting your dd down which I think you are already aware of.
STOP listening to his pseudo-psychology BULLSHIT, DO NOT let him stay at yours AT LEAST until he gets a job and grows the fuck up!
He's 26! At that age my parents were married with 3 DC, I was married and ttc, my brother was married with 1 DC, he is MORE than old enough to be PROPERLY out on his own. Finding a job (he is very fortunate has NO real restrictions on what job or hours he can do), paying his way, NOT being supported by others.
You CAN stop enabling him you are CHOOSING not to.
"He can have a mental illness ( made worse by the weed) AND be playing you. It’s not a case of either/or." Absolutely
"Would he join the forces? I see someone else has also suggested this." They wouldn't take him - what makes you think they would?! He'd be a bloody liability! Giving a guy who resists all authority and conventions of living and who has poor mental health inc an incident of self harm and likely has the potential to have a psychotic break at any time a gun? Come off it!!!
There HAS to come a point where "enough is enough" and the sooner that happens the better, for everyone!
Tell dd she is not to give him a key or let him in, she needs to block him on the phone etc
The ONLY way you have a chance of this working is if you do it in a fully committed way, anything else is going to make matters worse.