It's not that hard. The main issue is ensuring that there is adequate support where you install it and that the installation follows the specification for the fan, and then balancing the blades if necessary to prevent wobble. I've done a few of them and it's not rocket science. Depending on the size of the fan and the size of your sister, however, it might be difficult for her during stages of the installation.
Also, if your parents are still alive and there is a sizeable estate at stake, consider the wisdom of having your sister install a large motor with blades that hangs over your head.
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