It's the whole point of the game, it's not a harem game. Alexis is off-limits if you want to date her sister, or vice-versa. Just like with the good friends, if you're on the Layla love path then Elena will be off-limits, Valeria as well probably. You can't date Erynn and Alexis at the same time either.
If you date Alexis and only befriend Layla then there's no problem (although Erynn will still get pissed if you're trying to date her). Likewise if you don't do anything with Alexis and then date Layla then there's no problem.