Is "stepmother treatment" Indian English?

If you mean that in India it is a common, well-understood idiom meaning neglectful treatment, then no, in America it does not have any such well-established meaning. However, stepmothers get a pretty bad rap here in America too, so if you used it in a sentence, "I really got the stepmother treatment from my boss" or some such, I think people would get the idea. The phrase "wicked stepmother" is so commonly used in fairy tales that it is something of an idiom.

Here in the US it is "foster parent treatment" that is more prevalent. It is very common for foster parents to treat their bilogical children differently than their "income" children. With such a high divorce rate here in the US, practically everyone is a stepmother nowadays.

In Indian English, the word/phrase stepmotherly, with its variants step-motherly, and step motherly is so pervasive, that I initially thought that it was a part of BrE.

The word/phrase's etymology probably comes from the Hindi phrase 'Sauteli Ma', which means stepmother, and is commonly used in the language to denote an uncaring or oppressive treatment. There is a quote from the well known Indian film Sholay - "Main mausi hoon, sauteli ma nahin", which translates to "I am an (maternal) aunt, not a step mother".

Not all step mother's are bad, so the words/phrase do not make any sense in any country as lots of good families with step moms exists and the word " step mother treatment" got its popularly negative meaning/concept through movies & tales that spiced up it's stories.