One family could edge out in the other in prestige, business, or in a major competition. The other family calls foul play. It doesn’t even need to be the whole family: it could just be two people (usually people who are important/are doted on by important people). Anyway, the families cold-shoulder or murder each other. The extended family pledges revenge and the wound is never healed.
Yes and no.
You don’t need to explain everything in your world. I (as the reader) don’t need to know how the money is minted or the name of the guy who built the axel that was put on the wagon. I don’t even need to know the complicated science behind whatever science fiction (or fantasy) thing you may have come up with.
That being said, you need to give enough for it to make sense, or be such an amazingly fantastic writer that nobody will ever notice the holes. I have only read a few books where the writing was so fantastically good that I didn’t notice a single hole.
If it’s important to the plot, explain it in such a way that you give the audience what they need to understand it without opening up more questions. If it’s something that doesn’t actually work in real life (time travel, etc.) you are allowed to have the character say that they don’t know how it works, unless they would be in a position to actually know (i.e. scientist). Then, unless you have a really strong grasp of relativity and stuff like that, don’t try to make up an explanation. Just say that it works and accept that it will always be a hole.
A lot of stuff can be patched up with a line or two. If you’re in a post-apocalyptic society somewhere where you can’t get fossil fuel, you can get away with one line about how everything is now run on solar (or wind, tidal power, geothermal, etc.).
The point isn’t about filling up the entire world. It’s about telling the story in such a way that the world as you need to see it to understand is clear. Probably one of the best pieces of advice that I give is to talk it through with someone else. If feasible, turn on a voice recorder so you don’t lose any of it or have to stop to write stuff down, and talk through the world with a friend/colleague/etc. Tell them to ask you every question that they have about the world, and then make yourself answer the questions. And if you can’t answer it, you know you need to go back to it. Same with if they say it doesn’t make sense. If you have time, do this with more than one person. Different people with have different questions, and you will see different points of view.
All in all, though, don’t lose your story. Having a premise and plot that make sense, as well as good worldbuilding, are important, but it’s more important to write a story that you love (unless you’re under contract, in which case you should make sure you’re fulfilling your contract).
I hope that answered your question.
I just wanted you all to know that you can totally finish that piece that you’re working on, because you are super talented and wonderful and there are people that love you that would love to read your story, and you should totally do it.