It will definitely work on your Rebel XSi. You will also get a bonus in that the 150-600 fits both Full Frame and 1.6 size Canons. On your camera it is actually a 240mm-960mm. This is also the 3rd version of this lens and it performs even better then the previous versions. Make sure that you have a sturdy tripod and attach the lens to the tripod and not to the body.
Hope you enjoy it.