Great Grandma Was A Musician. A Lawyer Continues To Take Large Percentage Of Royalties.

My great-grandma was a Grammy-winning musician and passed a while ago. Her music remains relatively popular.
Her final heir, my grandfather, passed away a few years ago.
My mom and her three (3) siblings now receive the royalties—four (4) in total.
My mother and I recently discussed splits/breakdowns; she revealed the royalties are actually split five (5) ways.
A lawyer gets 20% of every check the family receives. She does not know if the lawyer was getting a split when Grandpa received them. She is certain the lawyer has received 20% of the money since the grandchildren have been.
Questions:
Does that sound right? We thought 20% seems high for a past musician.
Does the lawyer receive royalties in perpetuity?
What questions should my mother ask the lawyer/firm to ensure all is up and up?
Thanks for any feedback or input!
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