It'll be more powerful, yes, but I wouldn't call it overpowered.
A monster doesn't have to attack and provoke the Sentinels' reactions. It could take the dodge action each round. Watch the Sentinels whiff a bunch of attacks on this foe they're so intent on pinning down while its allies run amok on the party's back line. Or use an action that forces a saving throw. Or any allied monster could grapple the pinned creature and pull it out of one of the Sentinel's reach if you want to get kinda cheesey. Obviously, options 1 and 3 aren't going to work in solo monster fights, but those don't constitute even 10% of my fights, so I'd be okay with those off-times when the double Sentinels get to do their schtick without much interference.
As with any house rule, there will be side effects accompanying the modification, but the consequences you don't like can usually be handled with other adjustments. As I suggested, I'd start with monster tactics, but assuming reasonably intelligent foes, I wouldn't adjust their tactics til after the first exposure (unless they had foreknowledge of the PCs tactics). You can adjust the difficulty in other ways to account for the party's higher than average DPR if necessary, such as increased monster HP, or just higher CR monster groups than a PC party of their level would normally be tackling.
All that said, I wouldn't change the feat at my own table.