Hmm, I haven't read everything yet, so sorry if I am repeating what someone else has said. This is how I feel about it however.
I'll start with how I have explained it to the friends that DO know about me. If any of you have more than one child, you can probably relate to how you felt during the second pregnancy (whether you were the mother, or fater). I wanted another child, but I felt I was betraying my first child. What if I didn't love our second child as much, or in the same way? Was that fair? I didn't think it was possible to love another little person the same way I loved my first child. I laid awake worrying about this.
The day she was born, however, it all changed. I loved her the moment she was in my arms. I would have cried, but there was too much going on to process. All I knew was that it felt right to have her in my arms, and I was excited to get her older sister in the room to meet her. At that moment I realized I was not dividing or subtracting love from anyone. My heart expanded exponentially. I do love them both, and I love very different things about them, but I love them equally.
The same applies to the adult people who I love. I love them in different ways, for different reasons. I love them for exactly what they are. When I fall in love, I am not dividing anything. Except now maybe time since I have a new love interest.
Just as I spend time making sure each child knows they are loved equally (I have three now) I shall spend the same on each of my lovers.
As far as morality, I wrote off long ago that morality isn't what it used to be. It is an excuse, a weapon, to attack people of differing beliefs. True morality, will speak from your heart, not from you logical mind that can be trained and brainwashed by society.
As far as biblical defense, don't get me wrong. We own a Bible, we read it to our children, and Christianity is the core of our beliefs (which are ecclectic from many other religions). But we also realize that this book is written by man, who is numerous times stated to not be perfect. We use it as a guiding path, but don't take it word for word. It's been through many writings, translations and such. I always advise people to forge their OWN relationship with God, because he will speak to their heart and knows their desire.
When I listen to my heart, I feel very strongly that what I believe in is perfectly moral and just.
Something that occurred to me years ago when my husband was talking about his grandmother. Her first husband was killed at war. She was young, and remarried, and was widowed again at about 60 years of age. After that she formed a deep emotional bond with a man her age, who's wife was basically a vegetable.
Common belief is that you are rejoined with your loved ones in the afterlife. To whom will she be rejoined? What about the other men? Did she stop loving her first husband because he died? After all, in her eyes she will see him again.
I would advise your brother to set aside conventional morals. We live in a society controlled by media, and agressive religious figures who want to guilt you into their way of life. Break away and make your own decisions. This is your life, and it's limited. Don't spend it miserable conforming to someone elses social ideal, spend it pursiung what makes you happy. Life, liberty, and the pursuit of happiness is our constitutional right (ha, snuck that in there). No one but YOU gets to tell you what makes you happy.
I govern my life by this simple rule "And ye harm none, do as ye will." (yeah, did I mention ecclectic??)
*steps down from soapbox*