treat someone like a Dutch uncle - (infml., Am.) issue a frank, harsh, or severe comments and criticism to educate, encourage, or admonish someone.

- Pharos Afrikaans-English/English-Afrikaans Dictionary

Pharos facts:

Auntie (aunty) is used in South African English, as in a number of other African and Asian countries, not only to refer the sister of one’s mother or father: It is also used (mainly where there is an Afrikaans-speaking or African influence) by children as a sign of friendly respect to older women who are not related to the speaker (as in 'Give this to the auntie'). What is more particular to South Africa is that the word is not always used along with the name of the aunt. There are also two further uses of 'auntie' which go beyond the above. Firstly, as a derogatory reference to an old woman (as in 'That auntie has ugly teeth'); and secondly, to refer to the female owner of a shebeen (township tavern).

- Pharos Say Again? The other side of South African English