I think the real question your brother is asking is, "Why can't the girls I want work on MY terms?"
Isn't that right?
The (both) astrological and realistic answer to this question is--because everyone is different and they'll stick to their guns.
First thing's first. It's very important you told us you're a male model, because this opens up two doors: you're egotistical and want all kinds of different things, always keeping your options open (Sun conjunct Mercury conjunct Uranus, Venus and Mars in Aquarius), or the girls you seem to get with have a hard time accepting your complete inability to be stable.
Your Sun, Moon, Mercury, and Uranus are all in mutable fire signs. Yeah, you're hot and all (doesn't that boost your ego?) but that's ALL. It shows a very flexible, changable, detached attitude. Perhaps it isn't so much the people you attract, but that you can't maintain relations with the people you attract because they don't want the same things as you.
Your 5th house ruler is ruled by Venus in Aquarius (conjunct Mars by sign in the 8th house). So, you've got detached, sociable Venus in Aquarius conjunct detached and sociable Mars in Aquarius sextile Sun conjunct Mercury conjunct Uranus, adding even more detachment..and...the bottom line is...perhaps you want a substantial relationship, but you always end up detaching from it. It's not REALLY what you want or who you are. You like to keep your options open, remember? Whenever it gets too serious you seem to pull out.
The 7th house has Jupiter fallen in Capricorn in it. This house is ruled by Saturn in Scorpio in your 6th (I use whole sign houses). Saturn is about stability. In fact, it's in a FIXED sign. This means fixed, stable behavior. Your Saturn's square your Mars in Aquarius, the free, fun-loving sociable keep-my-options-open guy. It's also trine your Ascendant in Cancer, meaning you're open to the idea, but the bottom line is: YOU WON'T DO IT.
The change has to start with you. People see the image you project. You're this, you're that, you're ego, you're blahblahblah. Make these girls think otherwise, but don't do it to trick them, only do it if you're sincere. If you want long term, be sincere and open. If you want keep-my-options-open, be sincere and open. But you CAN'T have both!
Catatonia