Is "Cisternal Twins" an actual classification of twin types and if so, describe it
Cisternal doesn't make sense in this context, so either your mother is using an obscure term (quite possible, these things change a lot) or just has the wrong word. 

However, mirror twins are a known thing (and quite cool): http://www.twinsuk.co.uk/twinstips/4/140/twin-pregnancy--multiple-births/types-of-twins--identical-fraternal--unusual-twinning/.

She may have meant something like "chiral twins" or "cis-trans twins", which would be borrowing from a similar concept in chemistry to describe your mirror-image nature (but is, again, not a phrase I've heard in the twin literature.)