is my mum's cannabis smoking contributing to her behaviour?

[Brido]

No, her behaviour is acting like a responsible parent.
Your posting on twitter about harming yourself, of course she's then going to be worried sick and want to know what's happening especially if your not speaking to her about it.

Our daughter went through the same after my dad died, (he died in her arms). She started getting into fights, self harming and smoking. She was secretive about things online, and she was punished for changing her passwords without telling her mum, she lost all privileges regarding the internet, mobile phones etc, and we even had to keep her off school as she harmed herself on school premises after we had spoken to her teachers.

We were worried sick to even let her use the bathroom too long as she had written online to her Facebook friends that she was feeling suicidal.

Her older sister is 16 and for her behaviour for months on end was stopped the internet, she also changed her passwords and ended up grounded off and on for nearly a year before she finally started to learn that we weren't a piece of s*it on her shoes.

No, your mum's behaviour is normal, I'd be worried if she didn't do anything at all.