Im going to just lay out the order of operation here and ask if the triggers work out

1. Attack is made on player character's mount

2. Player character uses the Mounted Combatant feats passive ability to make the attack target them instead

3. Attack resolves

4. The player character can then react with an attack from the Sentinel Feat, even though the triggering attack had its target changed.

My logic to this is that the triggering attack question is similar to Sentinel and the Mirror Image spell. https://www.sageadvice.eu/2018/10/31/does-an-attacker-who-targets-a-mirror-image-instead-of-you-provoke-an-attack-from-the-sentinel-feat/

If you change the target of the attack from your mount to yourself, the enemy made an attack against you, not your mount. No Sentinel reaction.

So Sentinel is based on which target the attack resolves on? Makes sense then.