Forgiveness and Sacrifice (Sherlock's Daughter)

Who is it?

I hold the bottle higher as Asha hungrily devours the milk. It dribbles down her chin and onto my jeans. Her dark brown eyes staring up at me intently. Once finished I stand up and place her on my chest, swaying gently till she falls asleep.

After putting Asha in her cot, I wonder around the house aimlessly. I see mum's study, and obviously, I get curious. I've always been curious about her job.
Opening a draw, I begin to look at the files. Bills, letters and other boring things fill the pages. The drawers seem to hold nothing especially secret or interesting. As I put the files back, something falls out. Just another bill...

Maybe she's hidden the confidential stuff somewhere else... Maybe she's hiding something from me? What if she's lying? What if...
All sorts of things swirl around my head and I begin to panic. My breath quickens. Think rationally! Come on! I take ten, slow, deep breathes and go make myself a hot drink. Pull yourself together! Come on! You're being ridiculous. You're overanalysing!

I sit on the sofa and concentrate on my tea. It feels heavy, weighty. It warms up my cold hands, the smell is strong and pleasant. The mug has a chip in handle. Wait... Not just a 'I dropped my mug' chip... It's the letter 'M'. It's all fancy, swirling and looping. Must have been difficult to carve. Why would she do that for? Maybe it's from her company? No, they'd put the whole companies name on it. Mum wouldn't carve an 'M' into a mug and neither would Danny. I'm probably just getting suspicious about nothing.
My gaze skims the room. I see a photo of mum and Danny on their wedding day. The photo has fallen into a weird position. That's going to annoy me. After picking up the frame, I undo the back.
A note falls out. It reads:

The last patient is recovering.
Do something or you'll become one.
-M

-male writing
-HB pencil
-snapped midway through writing
- M??
- same M as the on the mug
- coincidence? The universe would rarely be so lazy
- Her company?? Malviola?

I take a picture of it on my phone. Someone is being threatened. Mum or Danny? I'll talk to Dad later, now I need to concentrate on acting normal. I can't look restless or suspicious. Act normal.
Who's 'M'? Who is it?