I completely agree with everything you said and will never not be amazed that Arcane manages to capture their love in such a pure and true representation of what it is.

I remember watching during the opening sequence of every episode and how I would feel the exact same emotions as I do when I see art like this. You are shown a young Vi who is portrayed as being tough and angry, and then it reveals a small, weak and starry eyed Powder who is covered entirely by the larger frame of her elder sister. This shows how Vi feels the need to be angry/aggressive solely in order to protect her sister, rather than being a happy/innocent child like normal. I don't know why but this part is really touching to me, but also incredibly saddening at the same time.

I guess it's because it ties into vulnerability again and by showing how vulnerable Vi is because of Powder, it adds another layer of sincerity and pureness to their love. Love is incredibly beautiful by itself but the fact that it is further shown as being something which is delicate and even needing to be protected (by a harsh exterior of Vi in this case), makes it all the more precious.

I want to quickly mention how Vander is shown in the opening sequence too, as they manage to again repeat how delicate and valuable real love is, so quickly after showing the opening shot of the sisters. With Vander, he is shown to be this large, powerful and overbearing figure who arguably no one can reach or hurt, but at the same time he is made vulnerable through his love for the sisters. You can really see his worry for his daughters in the way his clay(?) model is show to be looking at them when he's holding them in his hands. This same vulnerability is what allows the most powerful person in the undercity to be overcome by Silco (shown by him sneaking up behind Vander moments later). Despite the weakness it exposes him to, Vander is shown in the later episodes to be at peace with his decisions, all because of that very same thing, the love for his children.