If instructions are given orally about homework, ask the teacher to supplement this with written or other visual cues.
Simplify verbal directions and ask the child to repeat back in her own words what she is expected to do.
Slow the rate of speech when giving directions.
Establish a predictable routine for homework time so your niece knows what to expect.
Design homework time around several short 10 to 15 minute sessions rather than a prolonged work time.
Ultimately, however, if your niece and her mother are still not working together productively, it may be better to ask someone else to work with her rather than to jeopardize their relationship.