while this is a way to combat the lack of ingredients i just can't like this "oh btw you can go back to your world for a time" bit the author is dropping. It completely removes the weight of the sisters' decisions on which world to stay in after the miasma is cleared. Choice is really simple, if you like this new world at all then just stay there and you can go back every once in a while (maybe every few years?) visit people, grab anything you want/need and head back.
And if it turns out that you can only do this every 100 years or something like that it just means the lack of ingredients is going to show back up again later on