Be quite firm about her needs and yours as the person who takes the shortfall. I remember some 20 years ago when I was working with a group of adults with learning difficulties for adult education. The social workers were assessing transport needs etc. and one of the chaps had a long suffering brother who had promised his parents he would look after his brother. The carer brother had a history of saying "Yes" because he was uncomfortable with saying "No" and, despite being run ragged (his brother was also about in the night quite often) they were still contemplating asking him to transport one or two of the guys on a weekly basis - a time when he could have re-charged his batteries for a little while. It annoyed me how often they said "I expect ***** will do it and it was taken as sorted. Eventually, as you would expect, something snapped and reluctantly the labour of love became too much for the brother to handle. Many years on he is still quite a solitary person who never managed to carve out a social life for himself, even after his brother went into full-time residential care. It could all have been so different.
The point of this story is that social services will often let you do what you are prepared to do or can be co-erced into. I have not found anything changed much from personal experience.