Well, if you want your screenplay to be as close to the Catholic truth as possible, you will need to know some things…
Angels to start with are spirits of light, so unlike us they are not of a body and a soul, and they have no gender. I tell you this because you said “…] see their brothers and sister …]” assuming that there are male and female Angels, but there are not. Also, Angels don’t really call each other “brother” as they actually have no parents (They are creations of God, but even if they are of a superior nature than man, they are not his sons, unlike us).
Answering to your question: You may want to look up how the Catholic Church teaches the fall of Lucifer, who once was the most beautiful Angel but became the prince of darkness. God created every angel in existence at the very beginning of time, before he created anything else, and hasn’t created any other Angel since then, so right at the beginning God revealed to all the Angels that one day his Son would become flesh and that all of them should worship Him. Lucifer, the most beautiful Angel, the highest of all didn’t accept the Will of God, and as he was free to choice, he chose to “not serve” because he was not willing to worship a being of a lower nature than him (how could the most beautiful being of light worship a being of clay?), so he fell, together with many other Angels who decided to not serve as well, and they are known as the devil and the demons. Every Angel made their definitive decision to serve or not serve at the very beginning and since then no Angel, fallen or not, had change their mind.
Since Angels are spirits, they habit out of time and space. They actually can’t fall nor be tempted into the dark side as their decision was definitive and their love for God is not unstable as our fragile nature which habits in time and is exposed to mutation. That also is why fallen angels can’t be redeemed, and the same reason why hell is eternal (if you live out of time, you become unable to change your mind upon deciding to go to hell by your own free will).