But does that mean they are not intelligent sentient beings?
Dolphins do communicate with each other, they use vocal communication and body language. Our failure to understand it doesn't invalidate its existence.
They form lifelong familial bonds. They have friends and enemies and form complex social structures.
They play regularly and have advanced hunting tactics that require a high degree of coordination.
They even take recreational drugs.
I have spent a significant amount of time in the water with dolphins throughout my life, and I have developed relationships with dolphins I regularly see that are at the very least as complex as the one you have with the barrista that makes your coffee every day.
If the cutoff for intelligence is human intelligence, which human is the standard?
There's lots of grey area, and it all ends up being personal preference, but I think if one makes it a goal to always empathize it's a great starting point.
Oh yes, the slaughter of dolphins is tragic and wrong. Not because they are our 'brothers', but because they have value as part of the whole ecosystem.
I think its pretty cut-and-dried, dolphins are nowhere near as bright as humans.