You are right, it is completely unfair for family and friends to assume you have the ability to prevent your wife from using pot. I'm sorry they aren't more supportive.
So it sounds like your wife has no insight into her schizophrenia, a symptom which is also caused anosognosia. I'm going to recommend three resources that are a little off topic, but I hope they will give you some useful strategies.
1) A book called "I'm not sick! I don't need help!" by Xavier Amador. This book by a psychologist who specializes in researching anosognosia and has a brother with schizophrenia has been helpful to many family members. It has strategies in it for how to talk to someone who doesn't believe they are ill and how to convince them to do helpful things despite their lack of insight. You can buy it on amazon, and there is a video of the author talking
which has the basics.
2) Information on the BC Mental Health Act
. I'm assuming you and your wife live in BC? If so, this video is very helpful. It has information on involuntary treatment and extended leave, which are good things to know about if you have an ill wife with anosognosia.
3) Information on cannabis and psychosis
- yes, I know you know and believe her pot use isn't helpful for her schizophrenia, but here is validation, and is perhaps useful to friends and relatives.