Yep Princess Cadence is the granddaughter of Princess Luna...
Extra:
-PrincessLuna married Iresso (Human x Miraluka with human and Miraluka children)
-QueenCelestia married Adronikas (Twi'lek x Human and therefore had to adopt the sith pureblood)
-PrincesCelestia is too evil and badass to be married and have kids.
-LunarPrincess married Corso but didn't want kids out of fear of losing her carefree life as a smuggler
-PrinceSolaris originally married Kira but whenthe Jedi Order found out and dissaproved of it Solaris left while Kira stayed. Sometime later he met and married QueenLuna. Through the mysterious powers of the force they eventually had a Chiss daughter.
-PrincessCadence married Mako (damn you Bioware, I want this)
-SolarPrincess went on an adventure never to be seen again. Wether she married or not, only she knows.