It's no mistake that Octavia is there to witness almost every defining moment between Bellamy and Clarke.
Let’s think about this for a second…
Octavia was there by Bellamy’s side in the beginning when he was openly defiant against Clarke, even going as far as calling Clarke “princess”.
Octavia was there when Bellamy “took orders” from Clarke for the first time, when he was bandaging O’s leg, and Clarke came to ask if he really had a gun and told him to follow her and he did.
Octavia was there when Bellamy was torturing Lincoln, when he put his hand on Clarke’s shoulder and told her she didn’t have to watch.
Octavia was there when Clarke asked Bellamy to go with her on their day trip and she jokingly said it was because she didn’t want to be around someone she liked.
Octavia was there when Bellamy saved Clarke’s life on the bridge when her meeting with Anya went bad.
Octavia was there when Bellamy took Clarke’s side after the bridge when Finn was mad that they used guns.
Octavia was there when Clarke launched herself into Bellamy’s arms.
Octavia was there when Bellamy and Clarke had their heart to heart by the fire, and we know she wasn’t asleep because she joined the conversation effortlessly.
Octavia was there when Bellamy tossed the cup out of Clarke’s hand and stepped in front of her to protect her from a drawn sword.
Octavia was there when her brother admitted that it made sense that Gustus would do anything for Lexa, if it meant protecting her.
Octavia was there when Clarke told Bellamy that she was being weak and saw the look on Bellamy’s face when he realized what was happening.
This is not a coincidence! This is because the only person that knows Bellamy through and through is his sister. She’s the one that sees the change in Bellamy and she’s not afraid to comment on it (There’s something I thought I’d never see.” and “Look where that got him” come to mind.) Octavia is going to be the one that realizes Bellamy’s feelings for Clarke, probably before Bellamy does.
Right now, Bellamy is not in love with Clarke. He loves her, don’t get me wrong, he wants to be there for her and he wants to keep her safe, but he’s not quite to the point of wanting to be with her. But it will come, and I bet dollars to donuts that Octavia will know first, and probably say something about it, and that’s when he’ll make a move.