Your sister should NOT feed it another mouse so soon after it ate, as that will throw off the snake's digestion & likely cause it to regurgitate ALL it has just eaten. Not
only that, but then she (or you) will need to wait several weeks before feeding it again so that it has time to replenish digestive enzymes...otherwise, it may keep on
regurgitating meals & can even die from the resulting dehyhdration if you don't. PLEASE contact your sister before she feeds it again...that can cause a BIG set-back.
For best results with a very hungry & under-fed snake, don't feed big meals all at once anyway...it can easily be too much for it's stomach. Feeding is the important thing-
not sexing, & not running to the vet just for a check up (unless there's a real obvious problem). You want to allow it to settle in, eat, relax & repeat about 3 times BEFORE
you try to handle or take it anywhere. Do keep a watch for mites, just in case. But let it de-stress- new homes are SCARY for a snake. And a hungry snake can be scary
for you, so just slow down & be patient. Eating is job #1 for the snake, not handling. Stay patient, you'll get there, &for taking in a snake in need. Keep us posted?