Well, unfortunately, unless your sister (the mother) recognizes that there are problems, you can't do a whole lot.
At this point, she is refusing to think there are any problems. Frankly, that isn't uncommon..... pretty much everyone wants the perfect, healthy child... and admitting that your child has a problem can be a difficult step.
He probably needs an evaluation... my grandson just had an evaluation with a pediatric behavior specialist, and was diagnosed with Sensory Processing Disorder. One of the things we kept seeing was his sensitivity to noise.. he doesn't like loud noises, either... not to the point of screaming, but fearful, or covering his ears.
However, we saw these issues and the mother did, also... so she got the referral, and now has a referral for Physical Therapy.
How to get his mother to recognize this? Frankly, I don't know.