All the things 14+K
said are correct. Get Sandy spayed after kittens are weaned, about 11-12 wks. when they are ready to go to new homes, and do not let her outside until she is spayed
, as she can come into heat and get pregnant even when nursing.
Don't touch or move boxes, as this is an obvious trigger for her.
Do give Sandy some special
attention, don't just leave her locked in the room and put the food in without talking to her and telling her what a great mom she is and giving her lots of caresses. Give her some nice treats of chicken or salmon or whatever she likes. She will look forward to you coming in. Pick up the kittens and handle and stroke them at least twice a day. Her protectiveness is a natural instinct for kittens that age, and she knows better than your neighbour that her kittens should stay with her. She will tire of their antics when they're older, and she will not be as protective. I don't think she will have a grudge against you.