The third benefit of the Sentinel feat (PHB, p. 169-170) says:
When a creature within 5 feet of you makes an attack against a target other than you, you can use your reaction to make a melee weapon attack against the attacking creature.
The "reach" property of weapons applies to normal attacks, bonus attacks, and opportunity attacks, but this Sentinel attack is not explicitly any of those. Can I make this attack with my reaction against someone 10 feet away who makes an attack against a target other than me if I have a weapon with a 10-foot reach?