If my opponent plays Star Crossed on my Vex and his Scuttle Crab, but I react with my own Star Crossed and my Ravenbloom Student, and his Scuttle Crab will his Star Crossed no longer be valid due to his Scuttle Crab no longer being a valid target?

No, your opponent’s Star-Crossed will not "fizzle" or become invalid. It will still resolve and attempt to return any remaining valid targets to their owners' hands.

According to Rule 359.3.e.1, Rule 359.3.e.5, and Rule 359.3.e.8, spells in Riftbound resolve as much as possible even if some of their targets become illegal (e.g., if a unit is moved to a hand) while the spell is on the chain.

Sequence of Events

1. Opponent plays Star-Crossed: Targets your Vex, Apathetic and their Scuttle Crab. Star-Crossed is now on the chain.
2. You react with Star-Crossed: You target your Ravenbloom Student and their Scuttle Crab. Your Star-Crossed is now on top of the chain.
3. Chain State (Top to Bottom):
   • [Top] Your Star-Crossed (targets: Ravenbloom Student, Scuttle Crab)
   • [Bottom] Opponent’s Star-Crossed (targets: Vex, Apathetic, Scuttle Crab)
4. Resolution (LIFO - Last-In, First-Out):
   • Your Star-Crossed resolves first: It returns your Ravenbloom Student and their Scuttle Crab to their owners' hands.
   • Opponent's Star-Crossed resolves second: It checks its targets.
     • The Scuttle Crab is no longer on the board (it was returned to hand by your spell), so that target is now illegal.
     • However, your Vex, Apathetic is still on the board and is a legal target.
     • Because the spell has at least one remaining valid target (Vex), it proceeds to execute and returns Vex to your hand. The instruction to return the Scuttle Crab is simply ignored.

This interaction is consistent with the handling of multiple targets when one becomes unavailable during resolution, as seen in RiftJudge FAQ #10058 and #10311.