I really enjoyed HSC economics (at the time). But looking back at it, it's a very flawed and obscure course. And this is a prime example of why: we can't actually say anything about the impact of increasing CPI on MPC, because we need more information, such as the wage growth rate relative to inflation. So really, we can't give you a definite answer or anything like that.
But what your reasoning sounds fair, and I agree; given that all other variables (e.g. wage growth, consumption patterns, et cetera) are equal, an increase in the CPI should raise the MPC as the average household must spend more for the same basket of goods.
Edit: oh boy I did a nasty typo (originally I wrote that "an increase in the CPI should reduce the MPC...")