Nope, they aren't
Edit: I can see why you might think that since Alicia was in three Aerosmith Videos, and with Liv Tyler in one of them ('Crazy')
Silverstone won a leading part in the 1993 film The Crush, and Marty Callner who was directing the Aerosmith music video for 'Cryin'' , thought she would be perfect for the video. That's how she became know as the 'Aerosmith girl'
Is that movie the crush with Alicia Silverstone scary?
I've seen both, both are good. There is some suspense like Fear and Alicia Silverstone plays a bit of a stalker like Mark Wahlberg but not as scary. Definitely, worth watching though if you liked Fear Crush also has the girl who played Tara on Buffy the Vampire Slayer and Cary Elwes who has been in a lot but my favorite role was Robin Hood in Robin Hood: Men In Tights.
why did alicia silverstone replace rachel blanchard?
You have it backwards. Alicia Silverstone was first cast in the movie Clueless (1995) and then Rachel Blanchard was cast on the series (1996). I heard that Alicia didn't want to be mostly known as Cher in Clueless, so she turned down the part in the series. I actually thought that Alicia was A LOT better than Rachel as Cher.