As you rightly say, the sisters are not named in the Bible which means we have to look to other sources for this information. I think it's fair to say that overall we don't know for certain, but different churches have developed different teachings and traditions on this subject.
The Roman Catholic Church teach that the word "sisters" is used purely figuratively. They believe that Mary remained a virgin until her death, so Jesus had no full brothers or sisters (but possibly some half-siblings - children of Joseph with another partner).
The Eastern Orthodox Apostolic Tradition teaches that Jesus had a single (half-) sister called Salome and that it was she, not Mary, that was the mother of James and John. They also teach that Joseph had two sons, Jude and another James. They believe that Joseph's first wife died and he was 60-80 years old when he married Mary, having had these three children with his first wife.
Other denominations tend to take the usage of "sisters" at face value and believe that Jesus had more than one sister. The Bible refers to "all his sisters"; in English that would imply at least three (otherwise it would say "both his sisters" or "his sister") but the Greek word that's translated to "all" can mean two or more. Richard Bauckham, a Professor of New Testament Studies at the University of St. Andrews, tells us that...
Later Christian literature gives the names Mary and Salome to sisters
of Jesus. These names were extremely common Jewish women's names within
Palestine, but Salome seems not to have been used in the Jewish
diaspora. There is therefore some probability that the tradition of
these two names goes back to Palestinian Jewish Christian tradition,
and so it may be a reliable tradition.
However that "may be" is about as reliable as we can get.
Bauckham's essay The Relatives of Jesus makes for good further reading on this subject; it is well referenced and covers a wide range of viewpoints, including the Catholic and Orthodox ones I've mentioned above.
