When it comes to the straw hats witnessing the relationship between Luffy and his brothers the ones who are the most on focus are Zoro and Nami.
Zoro is the only straw hat who has met both Ace and Sabo and he has even commented on their similarities on how they worry about Luffy.
On chapter 259, when Ace visited Going Merry and talked with the straw hats, Zoro was on the front on both panels. He's the one with the exclamation mark which could indicate that he's our point of view on that scene:
On chapter 794, Sabo gives Luffy's vivre card to Zoro. And Zoro comments on the similarities between Ace and Sabo on how they talk about their little brother:
As for Nami, she's the one who sew Ace's vivre card on Luffy's hat so her captain would have his brother's card safely on him:
(chapter #160)
Then on chapter 490, she's the first one who questions Luffy what he's going to do about Ace's vivre card starting to diminish:
Also, she started to cry when she found out about Ace's death because she was worried about Luffy:
And she's the one who mentions the connection between Ace and Luffy to Tama. Also, she reassured the little girl that it was okay to talk about Ace with Luffy:
Bonus, both Nami and Zoro were the two straw hats who were the most impressed by Ace: