^^^ SSDI is Social Security Disability Insurance, Barsel, which tells us that the questioner is in the USA. (This website is based in the UK, Babylove276, but we welcome questions from all around the world. There won't be that many people around to answer your question right now though, as it's 1.30am here).
Forget the eBook idea. To write anything of quality will take many hundreds of hours (or more) and you might end up only making $50 from it. Writing an eBook as a hobby would be a great idea but writing to make any real money from it is basically a non-starter.
Perhaps you could start by telling your sister what other people pay for child care in the USA and getting her to improve her offer?
https://www.cnbc.com/2021/05/19/what-parents-spend-annually-on-child-care-costs-in-2021.html
Some people (who're able to spot designer goods and who know a bargain when they see one) seem to make decent money by buying stuff from charity shops (or do you call them 'thrift shops'?) and then selling on eBay or other selling sites (such as Ruby Lane). Others make things and sell them on Etsy (or similar).
More traditionally, people have made (and continue to make) money from home-based services such as clothes ironing.