Yes, {le} would work too.
But {lo ci mensi be mi} can, in context, be {lo ci mensi be mi [be'o poi kansa
mi'a]}. It would hardly ever imply that you may not have a fourth sister in the
future, for example, so it will normally be restricted to current sisters. Does
it suggest, without further context, that you currently have only
three sisters?
Yes, for there is no reason, without context, to be excluding some of them.
mu'o mi'e xorxes