I would be suggesting that your sister take herself and the baby to bed for a few days and they feed and sleep all day. Hopefully she will have someone who can make her food and bring her drinks, otherwise get some healthy snacks (saladas/cheese/tomotoes/cuecumbers/fruit... whatever takes her fancy) and some drink bottles and have it handy. So she is only concentrating on breakfast and tea.

Yes she can also express and topup with that. As for the weightloss... 40g isn't much depending on how she was weighed (eg different scales, if she had just emptied her bowels etc....)