The messy and disorganised part sounds exactly like my sister...
The mum yelling part too!
And I am also the exact opposite.
However, both my sister and I can multitask.
Being able to multitask has nothing to do with being messy or disorganised.
Although she can multitask well, she just does not choose any tasks to do with cleaning and organisation... makes sense?
In fact, I think my sister is much better at multitasking than me.
There is an iPhone game called Anthill that requires a high ability of multitasking to do well.
It is a warfare game where you need to manage and take care of a lot of things at the same time.
We have both completed the game and collected ALL the stars for every stage. 
I love the game.
If you try it, you'll get an idea about your multitasking ability.
There is a free version.
But it gets much harder in the later stages of the paid one.
Oh by the way, her bf does not like the same... maybe because he cannot multitask as he's male? Heehee...
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