'Every action has an equal and opposite reaction' which is simply, more modern words, basically means that your actions have consequences. When one does something bad, then something bad might just happen to them in return.
If a person decides to cheat on their partner, for example, there should absolutely be consequences to that. There should be a clear lesson the cheater should learn, which at the end of it they will hopefully understand that what they did is awful and should never happen again.
That is the lesson the man in this Reddit story tried to teach his cheating sister, and he did so by refusing to loan her the money she needed for her divorce lawyer. The brother insisted he cannot support his sister and her actions, and that she needs to understand that (say it with me folks) her actions have consequences. However, problems started to arose what the parents of the two got involved and stood by the sister's side, claiming they should not judge her for what she has done.
Keep scrolling for the full story. After you are done, check out this silly collection of funny Friends memes.