DD has painted her brother's face.

[HardLightHologram]

Using my makeup.

Doublewear, my Benefit pallet, a Lancôme eyeliner. Etc.

More makeup than I would get through in six months.

A few months ago I found the £40 cleanser I'd 'lost' in her bedroom, empty. As you can imagine, words were had. So it's not like she doesn't know not to use my stuff.

She has just screamed at me that if I don't want her to use my stuff I shouldn't keep it in the family bathroom.

She is twelve.

She has masses and masses of her own stuff but she has just told me she prefers mine because it's nicer. No idea why she felt the need to use my nice stuff as fucking face paint on ds.

I can't even look at her right now. She also tried to blame her brother. She painted his face, but tried to tell us he did it.

Other than keep it under lock and key, how the fuck do I deal with this?

I think the backstory is relevant because this isn't her first act of defiance like this and there is stress in the home. I'm blaming you, OP but it may well be contributing to her behaviour and therefore, her actions are not in isolation. Yes, 12 year olds are often awkward but this particular 12 year has a stressful background. One thing I have noticed is that money is spent a lot in your family; expensive make up, days out and much money seems to be spent on her. It doesn't sound like she appreciates this and I wonder why. How much time do you two get together? Not spa days or shopping days or fancy treats, just bonding time.