Review your own paper in Mathematics
It's absolutely not usual practice and a clear case of the editor in charge being asleep at the wheel. This should not be happening: An editor's job is to find impartial reviewers and asking an author (or even someone close to the author) is definitely failing at this job.

Just the same, it is unethical for you to accept such invitations. Politely point out that you are an author of the paper and that, therefore, it would clearly be a conflict of interest for you to review the paper.