Actually, it affects all the next generations coming. Those decisions will just continue til the end of time affecting all that comes in contact with any string created by your decision. Those that came before us made us who we are today. All of them together. Your decision will change and open a path that would not have been there had you not made the decision. Once that path has been opened, all that comes from it forever will have been affected by your decision. Everything each of us does will contribute to humanity. Even serial killers teach us all lessons to remember on how not to be. If we were all the same and had no challenges, would we learn anything? The decision itself may not be relevant in seven generations, but the effect is infinite.

I know my dad screwed up every one of us kids and everybody in our family that came and asked for his help. That's about 10 generations...

Is this why history is frequently rewritten. Commercial printing and music is more concerned with three month shelf life than preserving heritage wisdom. Where heritage artefacts are destroyed as quickly as possible to hide the inferiority of current manufacture. Water tables taking thousands of years to accumulate their precious content are destroyed through fracking or depleted by commerce that does not pay adequate compensation and also pollutes.
I live in a country that has a 50,000 year heritage. Governments have and still are allowing the destruction of that heritage whilst perpetrating thinly veiled genocide of First Nations Peoples. This is 2018 Australia.

the butterfly effect ie one decision can dramatically change everything quite immediately.

"A man has made a start at understanding the meaning of a human life when he plants trees the shade of which he knows he will never enjoy.". (mangled from memory)

7 is as good a number as any. I agree in principle. Another thought, anything taht affects you, may affect your offspring for the next 18 generations, and things that affect all 18 generations may make a genetic change. They are studying people who were at 911 for such affects.

On an evolutionary point of you somewhere it does make some sense.