Can one's karma be affected by the karma of another? Can one person's karma be affected by the karma of another person or other persons?
The short answer is “Yes”… Olivia Glad,… because the others provide the grounds, the right conditions for a kamma to bear fruit. How this comes to pass needs some explanation.

The laws of kamma are not just based on causes and effects, they depend on conditions. This is what prevents laws of kamma being deterministic. Understanding this would make it clear that one’s future is not determined by one’s past actions or kamma alone. Thus past kamma do not necessarily lead to kamma vipāka. 

What first gets created is a kammā beeja, or a kammā seed. For that kamma beeja to bring about its result (kamma vipāka), suitable conditions must exist. 

We can avoid many possible bad kamma vipāka by being mindful and not providing conditions for them to appear.  We can also make conditions for good kamma vipāka to bring about good results. If one goes into a bad neighborhood at night, that is providing conditions for any suitable bad kamma seeds waiting to bring about their bad vipāka.

Once a bad vipāka materializes, we can certainly manage it to minimize its effects, and in some cases even get rid of it by working to making conditions for a good kamma seed to overcome it. Kamma will not bring about any kamma vipāka, if and  when one attains the Sotapanna / Stream Entrant stage.  One can then erase all kamma vipāka that would lead to the four lower world realms.