Is it ok?! OF COURSE!!!
This can instantly cycle you for a specific amount of fish.
Take no more than 1/3 of her media. Then, you can add as much fish as 1/3 of her stocking amount. If you wait to add fish, then you risk the possibility of having the good bacteria die back.
Add her media, then run out and get some fish. Let's say your sister has 30 inches of fish in her tank, then you could add 10 inches of fish, fairly safely. You might want to consider going a bit less than that, but getting juvies (which is generally all that is available in the LFS anyway). Then let this go just like that for two full weeks to ensure everything is safe before adding more. 10 inches of juvenile fish should be a nice size shoal of some small fish.
What are your stocking plans?

Take no more than 1/3 of her media. Then, you can add as much fish as 1/3 of her stocking amount. If you wait to add fish, then you risk the possibility of having the good bacteria die back.
Add her media, then run out and get some fish. Let's say your sister has 30 inches of fish in her tank, then you could add 10 inches of fish, fairly safely. You might want to consider going a bit less than that, but getting juvies (which is generally all that is available in the LFS anyway). Then let this go just like that for two full weeks to ensure everything is safe before adding more. 10 inches of juvenile fish should be a nice size shoal of some small fish.
What are your stocking plans?