Not unless the attacking enemy is within 5 feet of you.
The third benefit of the Sentinel feat reads (PHB, p. 169-170):
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.
The description of the Echo Knight fighter's Manifest Echo feature says (EGtW, p. 183):
You can use the echo in the following ways:
- As a bonus action, you can teleport, magically swapping places with
your echo at a cost of 15 feet of your movement, regardless of the
distance between the two of you.
- When you take the Attack action on your turn, any attack you make
with that action can originate from your space or the echo’s space.
You make this choice for each attack.
- When a creature that you can see within 5 feet of your echo moves at
least 5 feet away from it, you can use your reaction to make an
opportunity attack against that creature as if you were in the echo’s
space.
The second bullet point of the Manifest Echo feature lets the Echo Knight attack from their echo's space when they take the Attack action. The third benefit lets them make an opportunity attack from the echo's space when a creature moves away from it.
However, neither Manifest Echo nor any other Echo Knight feature lets you make attacks from the echo's space in any other situation, as written. No feature lets your echo count as "you" for the purposes of other reactions that are based on a trigger occurring within a certain distance of you - so you can only take such a reaction if that trigger occurs within that distance of you (not just your echo).
As a result, the reaction from the third benefit of the Sentinel feat won't be triggered by an enemy within 5 feet of your echo who's attacking one of your allies - unless the attacking enemy is within 5 feet of you as well.
