So yeah, she does DEFINITELY know that Sakura is her sister. She just pretends that they aren't related as wished by their father, and as required by the charade that is their lives. However, she's pretty protective of Sakura, as shown by her main worry when that
random unimportant student gets stabbed. Her main line of dialogue there is "How will I look her in the eye tomorrow..." - big hints there as to her motivations.
As for not having a wish...come on...how many mages revealed their wishes straight up? Ever? Even Sabre didn't really learn Kiritsugu's wish from him directly. And Rin is a pretty self-conscious person.
Now, as for thinking Sabre is the most powerful servant? Who won the last War? Sabre. Who emerged as the most notable servant? Sabre. As far as Rin knows, Sabre class is the strongest straight-up fighter. And she's right, they are.
Traditionally, the three 'Knight' classes:
Sabre, Lancer, Archer
Are the most powerful in one-on-one combat.
The other classes:
Assassin, Berserker, Caster, Rider
All have various strengths and weaknesses but it is generally accepted that they are not as desirable in terms of up-front combat.
Obviously this is all highly academic. But Sabre is nonetheless widely regarded as the most powerful of the three Knight classes.
HOWEVER - you also need to bear in mind that Gilgamesh is an outlier. He's unique. He is LITERALLY one of the most powerful beings in the entire Nasuverse. Not just Fate, but the entire Nasuverse. He could have beaten everyone else, single-handed, in just one night in the Fourth War. Not even kidding. Word of God confirms it. XD
He's not normal. Most Archers aren't that powerful. Most Servants aren't anywhere near his caliber, full stop. He is literally the most overpowered, ridiculous character in the series. He just doesn't take any of it seriously and prefers to laugh as he watches everyone else run around the place.
And who said Kirei was teaching her anything about the Fourth war?