My sister has a son and she breastfeeds him. It’s been a few days since her son doesn’t eat well and keeps asking for breastmilk. Now that his daily nutrition almost only depends on my sister’s milk, she’s afraid that if she fasts it might affect his nutrition. Is it permissible for her to, for example, to fast every other day and eat well on other days so that she can be sure her son can have the right nutrition?