Only in some contexts. "Свои" is a possessive pronoun which refers back to the subject of a sentence. Without an actual sentence (and hence with no information about its subject), it sounds very odd.
A reflexive pronoun can't be used when there is no antecedent for it to refer back to.
So please explain why их матерей is not correct. Their + Genitive plural surely??
In the absence of any context, English "their mothers" can be translated to any case in Russian, depending of what kind of object it is in a particular sentence and what preposition precedes it. However, why would you treat it as an object in the absence of any context? "Их" is genitive regardless, since it indicates possession, but nominative "матери" would be the most natural choice here.