I had 4 children and I remember they used to get into squabbles, too. Its nicely referred to as 'Sibling Rivalry.' But, I'll tell you what I did not do. I refused to be a REFERREE! I would punish BOTH, or all THREE, if 3 were involved. Isn't this the same son who was not included in the "Teen" thingie at your church? He is fighting for attention, albeit, he is (like often happens) fighting for it in the wrong way. He just does not know he is doing that.
I think I'd find some small ways to give him a bit of special time and recognition. Like, invite only him to go to the store with Dad, or with you. That 15 year old daughter is already probably showing a few more signs of untying her own apron strings. Talking with friends more. Wanting to be with peers more. Things like that.
Whereas the 13 year old boy has not gotten that far along yet. Girls DO just naturally mature a little faster, and a 13 year old boy, in my opinion, is barely, BARELY, a teen. There is an age gap, and he just needs more time with Mom or Dad than she would right now, and he is resentful of big sister...who gets included more at church and perhaps in other ways, just due to her being a little older.
One other thing that I found to help at our house was to see the funny side of things as often as you can. At age 13, boys especially are FUNNY! They will annoy sister, just to be cute. If you and your husband can find ways to laugh with him more often, he will be getting the attention he craves...which that craving is pretty typical at his age. I don't mean laugh at him, but WITH him.
My 3 boys at that age I remember had pimples, which to them must have seemed like mountains on their noses. They'd ask for more Noxema cream, which I still say works! And I'd say, "Are you passing out Noxema to all the people you know who have pimples? "
They were developing stinky feet. I pretend to BEG them "PLEASE, please don't take off those shoes in this room! Run, everybody...run! He's taking them off!"
Their feet at that age were too big for their bodies, or so it seemed to me. I used to tease my boys of having clown feet. I'd' ask them, "Do those big shoes REALLY fit you? Are your feet THAT big?"
They seemed to trip over their own feet, and I'd say, "It's those big shoes you're wearing, boy!"
In all these ways, I was giving them attention, and laughing WITH them at the same time. It worked for me.
(Wouldn't it be nice if there was a "Mother's manual for Raising Boys" that we could buy?)
My favorite scripture: Malachi 3:16
"Then they that feared the LORD spake often one to another: and the LORD hearkened, and heard it, and a book of remembrance was written before him for them that feared the LORD, and that thought upon his name!" (Every time we speak of the Lord, or even THINK of him--its written down in a book of remembrance!)