iPhone saying sim not supported.

The phone is locked to a different carrier than the carrier of the SIM you're trying to use. Either use the same carrier your sister was, or contact the carrier the phone is locked to and ask them to unlock it.

How to unlock your iPhone for use with a different carrier - Apple Support

You will need find out which carrier your sister's phone is now locked to and ask them to unlock the phone. It would be better if your sister did this, since you are not the owner of the phone.

Then, contact your carrier and ask them to send you a new SIM card for the phone.