I think there a couple of things here. Habitual weed smokers do not have a problem with carrying weed around with them or smoking it anywhere. There is not a hint of it being inappropriate as they do not feel it is so.
Also attitudes towards weed is changing the world over and in reality UK are slightly behind the times - this is why there is such a high level of crime around it, if legalised there would be no need for county lines etc.
You can in fact get medical cannabis in the Uk prescribed and also there are now strains of high strength CBD and very low THC plants. I know that DSD in particular will only buy from her friend who grows these for that reason.
It is very challenging to try and use the but its illegal when even the police will walk past people smoking and not comment - they have too much pressure so someone smoking a joint to them is not worth the paperwork. Someone carrying quantity enough to distribute - totally different matter.
I think that you need to understand what you are uncomfortable with and set the boundaries that way.
Do you have alcohol in your home? If so it's challenging to use the don't want drugs as people who smoke weed will tell you that alcohol is far more damaging. Clearly its not, its all dependent on the user.
You say she just won't come around if challenged - how do you and her get on normally? I don't agree to just leave it to dad as this is your home. IF you have a good relationship with her then I don't see why you can't talk to her about how you feel about things she does in your home?
What if you don't confront but tell her why you feel uncomfortable - are you really worried about her being arrested or your home being raided? Valid and reasonable worries but not likely to happen.
My DS and my DSD (both adults) both smoke weed but after talking to them and doing on research we came to agreement so they can still come visit us but I have ground rules.
They all come for dinner on Sunday - they are not allowed to bring weed (I mean I don't search them but they are not allowed to get a joint out in front of me or sneak off to smoke it). They may have smoked it before and no doubt after but during family time on Sundays.
If its summer and we are having a bbq/get together in garden then if they ask me first sometimes I will let them bring it because they don't really want a drink (neither really drink)but again they have to check because they know there are certain family members that it would upset and out of respect they are not to do it.
I expect them to be mindful and respectful of where they smoke it as I know if I banned it outright then its a bigger battle. Are you generally anti cannabis or are you genuinely concerned? Does she suffer anxiety or smoke it for a reason in particular or is she just a stoner? DS suffers social anxiety and has CBD vapes that he will use if going out and about, would she consider this instead whilst she is at yours - there is no harm telling her if you don't understand it and are worried and don't like it being done around you and in your home.