Fiance's little sister begging for money for groceries

So my fiance and I are expecting our first child in 2 weeks. I'm unemployed and only he is working and we only have about $50 leftover after every paycheck because of all our bills. So we have been saving whatever we can to make sure we can buy baby stuff. Well, my fiances mom was an uber driver but her car got repo'd because she took out a loan and never payed for it so now she cant work because they took her car and she cant afford a rental anymore (she lives in California so that was her sole income) She called my fiance up and said things have been hard but she didnt ask us for money like she usually does. Shes always asking for money so we though it was weird. Well, today his little sister texts us through his moms phone begging for money for groceries and that his mom is frustrated and doesnt want to ask. We cant afford it at all but my fiance cant say no to his little sister. His older sister lives in cali too so we dont know why she didnt ask her for help but us. My fiance knows I dont like when he gives his mom money because she never pays us back and always spends it on stupid stuff. Plus, since shes unemployed now she will continue to ask for more money and we cant afford that. So we have no idea what to do because we have our own family to take care of now 😔 Also he gave her money 2 weeks ago to pay for her car rental but she doesnt have it anymore because she cant afford it (so where did that money go???)