No, it doesn't seem fair.
Factoring in not only the above, but also the expenses associated with each child.
Your sister is getting 50% GROSS of two average rates.

What's your NET on each child?
How much does the business (or YOU) pay for supplies (TP, paper towels, soap, disinfectant, etc) and food, water, electricity, gas, upkeep, etc?

You are flat out paying her that 50% and taking YOUR loss on the expenses part of the rate as well (but she's not taking ANY!).

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