Hi eveder,
I'm not sure why you would think that they are not sisters? Is it because they are so close in age? Or because Frances is not at home in 1841 when she should be 14?
There are lots of reasons why this could be - she could be out working as a servant. There are Frances Smiths of the right age living with other families in Kensington and St. James Clerkenwell, for example.
I see no reason why she would be the Frances Smith living with Ann Smith the laundress in 1841. Even if she was "adopted", she was baptised with the rest of the family back in 1830, so why then would you expect her to be living with her "real" family in 1841?
And the second daughter being named after the mother is not a clue, I don't think. The first daughter was often named after a grandmother, so that could be where the Palmer comes from.
Does Frances get married? Who does she name as her father on her marriage certificate?
Hope that is useful! If you would like anything else looked up, just give us a shout!

MJP