You love your brother because you are willing to help him through thick and thin, and he will do the same for you. However, you are in love with someone when not only that but you also find them irresistibly sexy and your heart goes pitapat just thinking about them.
I look at it as you can love someone like your brother, parents, close friend; but it doesn’t mean you want to have karnal knowledge with them.
But when you are in love, your brain has created a new sub-universe, and it is dancing in the joy of that person; it is similar to an addiction, like the first time you get high (on something) its wonderful and you want more of that joy.