Hi Serenade,
I am speaking from my personal experiences with my girl who is aslo End Dec kid, and whose Mandarin was terrible initially but is showing progress now.
Frnakly, I am surprise that the pre-school teachers asked your sister to sign your niece up for drawing and Chinese enrichment classes. Your niece is only 3 1/2 years, she should be learning all these basics in her nursery.There shouldn't be any need to to for such enrichment classes at this stage. Is her class too big that the teacher cannot afford to spend more time,give your niece more attention when she needs help?
I would say my girl's pre-school teacher did a wonderful job of reading and making her read a whole series of Chinese books.Besides, I started a Speak Mandarin Days for my girl at home, every Mon,Wed,Fri, we will speak to her in Mandarin at home.
She may not be as fluent as those who are from Mandarin Speaking homes, but at least she doesn't ignore people who spoke to her in Mandarin, like she did before.
Ask your sister to look around for other pre-schools and compare their curriculum,styles,systems.
I agree that Phonics is important, preferably they should start latest from nursery, if not playgroup.My girl started phonics when she's two.
Hanyu Pinyin is better to defer until after they have learned English phonics to avoid confusing them.