kita-no problem..and welcome to A2K.
Yes, they are sisters--one from the first litter of the year, one from the last...so, not typical sisters but, sisters still.
They are very hard to photograph and get any detail because they are so very black..plus they both have really great shiny coats. Doesnt help with pics but, is lovely to have.
Black cats themselves are pretty interesting.
I read somewhere that domestic black cats without controlled breeding are the most populous of the colors. Kinda like a combo of all the colors, happening most often.
My gals are a bit odd...they are solid black with shadow tabby--but thats just a recessive gene thing
(if you're interested in all that stuff...heres a link and this page has another Genetic Code page if you're really interested
http://www.fanciers.com/other-faqs/colors.html#common_colors )