It was all a problem of categorization, Peter realized one day with sudden clarity.  He'd slotted Claire into certain categories after they'd met and some of those categories had been mutually exclusive. 

Girl he saved - that was a given.  Save the cheerleader, save the world.  God, that seemed like so long ago. 

Girl who'd saved him was next on the list.  He smiled at the memory of waking up to see Claire's relieved face above him, her hand holding a bloody shard of glass. 

Best friend - they'd grown into that one gradually.   

Niece.

Peter took a deep breath and leaned against the ledge of the Deveaux building roof.  He came up here sometimes to think and he'd been doing a lot of thinking lately.

Niece was the categorization, forced upon him by fate, that had knocked so many others out of the running.  Sure, he'd been with Simone at the time, however briefly, and yes Claire had only been in high school, but it was hard to deny the connection they felt with each other when they met.  

"When I met you I finally felt like I was part of something," she had said once.  He'd replied, "It's funny. I felt the same thing when I met you."

That was a category that almost got dropped until he was able to rationalize that there could be friends that were soulmates.  That had to be possible, he'd assured himself.  Plus, he couldn't think of another category that covered the same feelings.  He'd tried to slot it into Family, but that never felt quite right.  Best friend was close, but didn't completely cover it.  He'd asked Claire about soulmates being just friends once (not mentioning how it concerned them) and after a slight frown she'd gone on to elaborate the intricacies of Joey and Dawson's relationship on Dawson's Creek. 

Now it turned out they weren't related and Peter wasn't quite sure how to handle it.   

He'd taken to staring at her for long periods of time, as if just looking at her face would help him piece his broken life together the same way thinking of her face had saved his life once upon a time. 

He also knew that she thought he was thinking about his mother and his brother (could he even call them that anymore?) when he was really thinking about her.  He should've been thinking about Nathan who was no longer his brother and Simon and Monty who were no longer his nephews, but all he could think about was Claire who was no longer his niece. 

When he thinks of Claire after his mother's sudden revelation, he doesn't feel pain.  All he can feel is relief.
He let that relief flood him as the sun went down and then he headed back to his apartment for another night alone.  As he crawled into bed he told himself life wouldn't always be this complicated, and he almost let himself believe it.

One time, about a month after he discovered they weren't related, he looked over at Claire and she smiled at him and he couldn't help but smile back.  He found himself thinking, "I could kiss her now if I wanted to."

When he apologized and muttered something about it being a reflex, she looked confused but didn't ask him to elaborate.

"A leaf," he said in response to her questioning glance.  He opened his hand and let the piece of foliage fall to the ground.  He wished he could touch her hair without having an excuse. 

He started to let himself think about her in ways he'd never let himself before.  He's imagined kissing her without feeling guilty.

Suddenly, he became the one touching her more instead of the other way around.  He constantly put his arm around her on movie nights and whenever she leaned into him he wondered "What if?"  He brushed strands of hair out of her face and pressed his hand against her upper arm whenever he wanted to point something out. 

Peter spent more time on the rooftop of the Deveaux building thinking lately.  Most of the time he thought about her.  One night while he sat up there and watched the sun go down, he realized that he loved her and not in the way he thought he did before when they were related.  He really loved her and didn't know what the hell to do about it.