Will Ghost of Tsushima be like Sekiro?

Yes. Ghost of Tsushima is like Sekiro in the context that they have similarities, not just in the theme of a Samurai-Ninja duality, but on the story and combat as well.

This does not mean though that Ghost is like a Souls game, because it is not. We answered this question already here - Is Ghost of Tsushima as hard as Sekiro? and here - Is Ghost of Tsushima like Dark Souls?

Our focus now is on what makes them similar.

Sekiro and Ghost of Tsushima Similarities

They are both set in Feudal Japan

The first obvious similarity is the setting. They are both set in feudal Japan, when Samurai were present in society, ruling with power and influence.

This means you can expect a core set of enemies you will face in both games are Samurai as well, in fact, the strongest bosses in Sekiro are all Samurai, excluding the monstrosities.

The Main Weapon of Both is a Katana

Both characters can use different kinds of tools for combat, but their primary weapon is a Katana. So this will not be like the other Souls games where you can use any weapon you like. This time, it's Dex build all the time. You have to master that Katana.

Both can fight as a Samurai and as a Ninja

This is the strongest thematic similarity between the two. You can fight as a Samurai in Sekiro and avoid using any Shinobi attacks. Similarly, Jin can switch to both combat styles.

Both Have Deep Combat Mechanics

Stealth

Both games allow the characters to crouch and hide from enemies. This typically leads to assassination.

Grappling Hook

Both characters can traverse wide gaps with their grappling hooks.

Assassination

Both have mechanics for assassination.

Parrying

Both enable the character to parry the enemy for a deadly counter-attack.

Blocking

You can block attacks in both games. 

Dodging

You can dodge attacks in both games.

Perfect Parrying

And with proper upgrades, Ghost also allows you to do a Perfect Parry.