A sibling who eats everything
Serve the same table, and let each child eat what they eat. Comparing them out loud makes the careful eater the problem.
No public comparison
The child who is learning still gets their familiar food. The child who eats widely does not need to be the example.
“Look at your brother” at the table is a speech the careful eater will remember longer than the vegetable.
Same table, different plates
Both can have rice. One can have a wider set of foods. The learning child still gets one new food in a small amount, not the sibling’s whole menu.
Allergies can differ. Serve each plate from what that child can have. Cheese stays off the plate where milk is an allergy.
Praise in private if you praise at all
You do not have to praise eating. If you want to notice something, notice sitting together, after the meal, without a score.
The wide eater is not the standard the other child has to meet this year.
A sentence you can say
“Your rice is here. His plate is his.”