To calculate Taylor's sister's commission, you can use the formula:
\[ \text{Commission} = \text{Sales Amount} \times \text{Commission Rate} \]
In this case, her sales amount is $900, and her commission rate is 2.5% (which can be expressed as 0.025 in decimal form).
Now, plug in the values:
\[ \text{Commission} = 900 \times 0.025 \]
Calculating this gives:
\[ \text{Commission} = 22.5 \]
Therefore, Taylor's sister's commission for last week is $22.50.