Can this fandom just...not? For one week, just stop blowing everything up into a huge crisis?
First of all, the sneak peek ends after that line. We don’t know how Magnus replies, we don’t know if he challenges that line, we don’t know if Alec accepts or acknowledges that challenge. The context we have is limited. So relax.
Second of all, this is Alec. This is what Alec does. He defines himself by his ability to protect the people he loves and he will sacrifice anything for them. That is who he is.
Whether or not Jace is worthy of that love is a different discussion, and not particularly relevant right now, because whether we as fans like Jace or not doesn’t matter.
What matters is the characters being written consistently, and this is 1000% consistent with everything we know about Alec. Alec wouldn’t be Alec if he weren’t ready to put it all on the line to save someone he loves. He’d do the same for Magnus or Izzy.
To Alec, there is no choice here. Someone he loves is lost, he has to find them, end of. It’s not a statement about who is the most important to Alec, it’s just Alec being Alec. He doesn’t love by half-measures, not Jace, not Magnus, not anyone. He loves them all with everything he has and will stop at nothing to save them and that’s just...that’s just all there is to it.
It doesn’t need to be more dramatic than that.