Women who do the same crimes also get lesser sentences, so you could argue that it is sexist using the same logic to argue racism. It could be more sexist than it is racist, if you take into account that the disparity between black and white men is due to their own actions whereas the disparity between men and women could be much more due to external forces outside their control.
However, overall, the issues where there are more black men in prisons and less women in general are due to much more nuanced factors than "racism" or "sexism" in the system. Granted, those things could have a factor, but I think that focusing on these things and not looking at the real problems will reduce the efficacy of change to reduce our crime rate. Individually, people are more than their race and their sex so we really should look further into their cases and realize that, if those things were changed, they could still be in prison because that's not what is making them criminals.