There's people making conclusion based on the system as used by Vampire The Masquerade. This game however is not based on VTM, but like most vampire themed games there will be similarities.
First generation would mean the first vampire and certainly the oldest and least humane, but also the strongest of them all.
In VTM the strongest vampires are 3rd generation and are called the Antediluvians (they predate the biblical big flood)
The 4th generation are the methuselahs and are either child of the Antediluvians or committed diablerie (feeding on and killing to take another vampires powers) to become one.
Is it possible to become more powerful then a generation 1 vampire? By VTM rules the maximum is 3rd generation but that's inherent to the lore ; Cain is first generation, created 3 childs (2nd gen) that in their turn created 13 servants (3rd gen) that revolted and killed their masters when Cain left, after Cain returned he cursed the 3rd generation with several of the vampiric traits we know of like weakness to daylight, etc. From that perspective 3rd gen are the first "true" vampires...
Vampires powers grow with age, but it's possible to grow by means of diablerie aswell. However it's simply not possible to become stronger then the first generation. Not based on the VTM rules since those represent a game statistics model.
But in theory ofcourse aslong you keep killing vampires for their powers eventually you'll be stronger then the first generation although you might be the only one left by the time you reach that point....
Actually the game mechanics are heavily inspired by VtM.
While
c3p0 thought I was going to do a multi page answer to another question, I am a little bit under the weather the last couple of days so I will just say this for this post and then go back to resting up.
The Sisters are Goddesses. They are each very very powerful. Whether they represent Caine from VtM power vamps or something more or something less, it really doesn't matter. For purposes of our MC's power, they are the founders.
Now a 7th generation vampire is the first vampire in VtM which can break the 5 dot ceiling on attributes and powers and such, and in theory are powerful enough to found their own bloodlines. These bloodlines are derivatives of the main line from the clan founder. They are not, ever, as powerful as the original founder of the line.
2 things, as has been stated above go into the "power" of the vampire. Blood and Time.
This breaks down somewhat when the vampire approaches the founder's level in the bloodline.
As I mentioned, you can look in the second edition VtM book Elysium for rules on creating Elder Powered fledgling vampires.
That is what the MC is. Much more powerful that time would indicate, and time can never surpass certain limits on further removed vampires ie 13th generation vampire that is 1000 years old would be very tough, but is still capped by the 5 dot rule. a 5th generation vampire can have 8 dots. If that baby vampire is a combat god, then he will wipe the floor with the older but less potent blooded vampire. Plus a number of disciplines for mind control simply will not work on the younger but more potent vampire, no matter how practiced the geriatric 13th generation vampire is.
Sooooooo
The MC is in a class that is far above his age. He is capable of destroying most any vampire he will meet in the story, but he is not the most powerful vampire in the game. A number of other vampires will exceed in potency and age.
Additionally-
Diablerie does not directly take you to the level of the vampire you slurp down, necessarily. It can vary a great deal in what you get. You certainly will lower your generation, but as to how much? That is a good question. If you are 8th and devour a 7th, you in fact become a 7th. If you are 7th devouring a 6th then yes you lower to 6th. If you are 7th and devour a 1st somehow, you likely become a sock puppet for the first generation, and lower a generation or three for your troubles.
Hope this helps, but to reiterate, the mechanics of the game are very similar to VtM, but the setting, powers, and mythology are all the author's creation.
Peace