Deflect Missiles works on ranged attacks targeted at you. The Sentinel feat does absolutely nothing to change that. You seem to be misreading the following part from Sentinel:
When a creature within 5 feet of you makes an attack against a target other than you (and that target doesn’t have this feat), you can use your reaction to make a melee weapon attack against the attacking creature.
A ranged weapon's ammunition hitting somebody is not "a creature attacking somebody", so nothing in the rules will allow you to attack the ammunition.
However, if a character with a ranged weapon attacks somebody other than you, and you are standing next to them, you will get to use your reaction to attack them. (But their attack will still go through, you do not stop it.)
Using Deflect Missile to protect somebody else sounds like a great, fun way to flavor the ability more, but you're entirely in homebrew territory, it is not something supported by the rules.