A
little absurb but I shall give you the benefit of doubt.
It is pleasing that you still retain some sanity to be able to
identify the problem. You mentioned that your bf is serving the
nation and you've little chance to see him.
And your brother is nicer to you nowadays, not to mention
gentlemanly and more mature. I'd not say that you are taking your
brother as a subsitute for your bf.
In whatever case, you know it is wrong to harbour such feelings for
a sibling - then rectify the problem. You may see him as a figure
of admiration for his qualities. After all, he is your brother, you
may adore him and love him as a brother.
Simple solution to a simple problem, you just got to jump out of
it. If you still find yourself trapped in such a situation after
sometime, it is then best to seek professional help.