From what I have read. (most of if not all my information is 5e) When thinbloods diablarises a full kindred. They become a full kindred of one lower generation and the victim's clan. (this depends on if they are successful or not. Like all diablerie.) They lose their thinblood merits. (Day drinker, thinblood alchemy and all that.) They also lose their thinblood flaws. (vite dependency, dead flesh and all that.)
Discipline affinity. Depending on your storyteller you should be able to keep it. You will likely have spent EXP on it. You might even be able to spend EXP from the diablerie on it if the diablarised Kindred is proficient in it.
Now for a thinblood diabolizing another thinblood. Mechanically thinblood have zero in blood potency. So it becomes a humanity V resolve roll. (the one committing diablerie rolls humanity the victim rolls to resolve.) for every success. you get five EXP to spend immediately on Blood potency (up to the victim's blood potency. So zero.) or a discipline they are proficient with. As thin blood alchemy is treated as a discipline I would argue you could spend those point on it. Same with your discipline from discipline affinity. (it may be up to the Storyteller whether the Thinblood alchemy's distillation method effects this.)
The rules are written in a way that allows room for interpretation. I hope you have fun and that this helps.
If I remember correctly, the thin-blooded diablerist keeps his disciplines; he may even increase them to their maximum with XP. He can't learn any more disciplines. Of course, The Prince of the city may very well hold a summary trial and have him executed in as convenient a fashion as possible.