Only do this if you have the means to pay that mortgage yourself, imo.

You and your sister are taking all of the risk here. Your friends could lose their job or simply just move after a short time and then you are on the hook to find more tenets. I suppose you could just sell it if they move but you may lose money.

You would be a great friend to do that for them so that you could essentially lock them into a fixed rent for many years, but understand that it is not without risk.