Is it permissible to shave a beard that grows thickly on the sides, with sparse hair on the chin, to please family members who see it as incomplete, knowing that they ask for it to be shaved once so it may grow thickly?

Do not obey your mother and sisters in shaving your beard, not even once; because shaving it is a violation of the Prophet's command - may Allah's prayers and peace be upon him - to let it grow. He said: "Trim the moustaches and let the beards grow." And the fact that the beard is incomplete in its initial appearance is normal, and it will become complete, by Allah's will.