Step-by-step explanation:
Based on the context (since you are writing to an elder brother), the letter format should be an informal letter format. A sample letter could read;
"Dear Brother,
I'm happy to communicate with you after such a long time. I hope you are doing great bro where you are.
I'm writing to inform you about the poor condition our family is in presently, due to the fact that you have not been providing the necessary financial support needed by the family.
Also,..."