if i was a level 5 sorcerer and put ten levels inro a prestige class that grants new spells every level. so then i would be character level 15. so if i was putting levels into sorceror again, would i get spells as if i was level 16 or level 6
If the prestige class grants spells each level, then your spells would be as a 16th level Sorcerer, but your Blood Line Powers and Blood Line Feats would be as a 6th Sorcerer.
What Cid said, the Prestige Class would only give you Effective Caster Levels unless it said otherwise. So, effects, like a familiar if you had one, would be that of a 6th level Sorcerer as far as its special abilities as the chart lists Master Class Level, not Caster Level. However, the Improved Familiar feat, for example, requires different caster levels so you would meet those requirements.
Keep Class levels and Caster levels separate and you should be fine. Just look at what feats and other things require and compare it to the correct level.