Hello everyone, long time lurker here just looking to shed what light I can on this situation.
The Prima Guide for Late Night suggests that your sims' astrological signs do not play a huge part, apparantly each sign has four compatible other signs, and upon discovering another sim's sign that is compatible with your own you gain a small relationship bonus, and that is all. No changes to personality, etc, as that is what traits are for. All sign compatibility is positive, there are no negative effects.
Sign compatibilty is:
Arius: Leo, Sagittarius, Gemini, Aquarius
Taurus: Virgo, Capricorn, Cancer, Pisces
Gemini: Libra, Aquarius, Aries, Leo
Cancer: Scorpio, Pisces, Tarus, Virgo
Leo: Aries, Sagittarius, Gemini, Libra
Virgo: Taurus, Capricorn, Cancer, Scorpio
Libra: Gemini, Aquarius, Leo, Sagittarius
Scorpio: Cancer, Pisces, Virgo, Capricorn
Sagittarius: Leo, Aries, Libra, Aquarius
Capricorn: Virgo, Taurus, Scorpio, Pisces
Aquarius: Libra, Gemini, Sagittarius, Aries
Pisces: Scorpio, Cancer, Capricorn, Taurus
Now I can't personally confirm the accuracy of this information, Prima somethings gets things wrong, but they get a lot of things right too, so I wouldn't be surprised if this is too.
It doesn't make much sense to me personally for the signs to have any personality change, so I'm glad if this is true, because that somewhat circumvents the trait system.