'Both women are sisters' is pretty awkward (in English at least) for saying that the two women are daughters of the same parent. 'Both women are sisters' implies that each of the two women have at least one sibling but that they are not sisters.... I cannot help wonder whether 'Die beiden Frauen sind Schwestern' truly means that they are two daughters of the same parent?
Comment made by Katherle:
The endings of "beide" are the same as those for adjectives, i.e. among other things they depend on whether there is an article before the word "beide" or not: "Die beiden Frauen sind Schwestern " vs. "Beide Frauen sind Schwestern ".
Background information on adjective endings: http://coerll.utexas.edu/gg/gr/index.html?si=adj
die beiden Frauen is correct, beiden is weakly inflected and Frauen is a plural. Weakly inflected plurals adjectives ends with -en in both the nominative and accusative cases.
die beide Frauen is incorrect since beide is (almost?) always followed by a plural. beide Frauen (without a die) would be correct in the accusative case though since beide is mixed inflected here.