Ask the title says I am curious on why royal family would fight against eachother? Didn't they know that such infighting would lead to their downfall? Or insteading fighting one another the help eachother with their goals? They may not be king but they would still be royalty and have a good life. And also are they're any examples of royal families being strongly united together. Kind of like the Starks in a Song of Ice and Fire.
Sorry, we don't allow "throughout history" questions. It's not that your question was bad; it's that these kinds of questions tend to produce threads that are collections of disjointed, partial, inadequate responses. If you have a question about a specific era, like "classical Americas" or "ancient and medieval Asia", feel free to rewrite your question and submit it again. If you don't want to rewrite it, you might try submitting it to /r/history, /r/askhistory, or /r/tellmeafact. I'd recommend /r/history for this one. :)
Good luck!