My SIL earns a small amount of money writing online. In around 2 years she has earned around US$15k. It's all still in PayPal as the 'reason' is that the fees to convert it an take it out are very high. While that is true, there isn't any other alternative one can do! It's been sitting there for around 2 years and DH now has realized she is doing what her dad did (he was unemployed for many years and would fantastically save his 'pocket money' that he got from DH's mum and ask for additional money for buying groceries etc). Which is why after they divorced and he got a 100k settlement for the house, that wasn't the only money he had despite not working for many years, he had additional savings (which he placed in ISAs and bonds) which is why he was able to retire to Thailand (and also support a new wife)...
It's not my business what she does with her money but I dont think she is aware of the implications of leaving it in PayPal. Firstly, inflation. Secondly, monies in PayPal are not guaranteed. She is not contributing NI so no state pension. I suggested contributing to a pension or buying bonds/shares, which she laughed at and said it was silly cos 'mummy has no pension too why should I have one'. 'Well mummy has a state pension, tiny private pension and owns her house. That's very different.' SIL lives with MIL which is fine for now but if MIL needs care, what happens then?
I am concerned about this as DH's other siblings are overseas, I don't think there is any plan for the future so DH and I might be supporting her. I realized this yesterday when DH's sister blew up at DH and told him that because DH and their siblings left her out of playtime, he owes her. we think MIL might be encouraging this narrative perhaps not consciously but this narrative is quite convenient as MIL doesn't need to solve the existing problem (SIL was diagnosed but has never received medical help and to this day denies there is a problem) and SIL would always be taken care of. If we are to support her, then it's easy if there is money (just like FIL had money). Also as she is young, there are many years left where the problem can be solved but I fear the problem might get worse since everyone is ignoring it..