You have no official income, and would like to get a visa to come to UK to stay for six month, and asking which documents need to be submitted.
Unfortunately, in my opinion, no document you can submit in those circumstances would qualify you for a 6 month stay (but see below).
First, for UK it seems that good financial standing of a potential visitor is one of the most important criteria to receive the visa (unlike USA, where past travel history seems to be more important). You have no official income and can leave for six months, which means to ECO that you have really no real ties to Philippines. The logic is that people with ties to the country (family, job, weddings to attend, studies to finish) can't really leave for six months, and in your situation you are more likely to overstay. A promise from you not to overstay is unfortunately useless. Please read this excellent answer of why bank statements are asked for - it covers your case too.
Second, considering that you have no resources of your own and totally depend on sponsor family, the potential for you to end up being abused (for example forced to work illegally) is high. This is not taken lightly, and it is another concern you need to overcome.
So the only way I could see it possibly working is if your sister's husband is really, really rich (not just "doing well" - we're talking a millionaire here). In this case it would be relatively easy to overcome a presumption you're coming as a free babysitter - especially if he provides support document which confirms that he already retained a babysitter agency. Then he should be able to find a solicitor which would help to prepare your application - this is also expensive and thus would serve as further evidence you are not coming to earn money illegally. In this case you have some chances - but still no guarantee.
Unless this is the case, I would recommend not to apply. This may sound harsh, but if you apply and are refused now, you will have more difficulty applying later, even if your circumstances improve.