All right, I decided to flesh this out into the full answer...
The seven sisters of the Pleiades
You may see me in 7 if it's your time
In the Winter, with good eyesight, you may count all seven. This requires, of course, a clear night, as well as allowing your eyes to become accustomed to darker light to see the dimmest of the seven stars. Furthermore, in November, the stars remain visible from sunset to sunrise.
Though you will probably see me in 6 all around town
The Subaru logo is of the constellation Pleiades (Subaru is the Japanese word for Pleiades) but only has 6 stars in it. You will likely see a Subaru car when you go anywhere
Alternate explanation (not intended meaning): In town, bright light will make it hard to see all 7. Most cultures refer to the 7th sister as being missing, forced to sit in the back, or have some other explanation as to why only 6 are (easily) visible.
Some say my predecessor can hold up the world
As Eutherpy notes, they are the offspring of Atlas, who holds up the world.
In the west I am a group of siblings in a cow
Those 7 stars form the seven sisters (siblings) of the Pleiades (in Taurus, the cow) in Western astronomy.
Though in the east, a furry skull house in an achromatic cat
In Chinese astronomy, there is a group of stars that forms the White tiger of the West (achromatic cat). Seven of those stars form the 昴 (translated to hairy head). Groups of the stars are translated (again, according to the Wiki linked) as a mansion. Thus, achromatic cat's furry skull house.
And of course, there's the title
Horoscope relates to astronomy