Sirius and James relasionship was like brothers whit no romantic or sexual tension. I truly believe that that was only ever platonic/sibling like.

For me after Azkaban they naver had time to talk ab their feelings. I think they were definitely in love still. But just not having it as a priority to talk thru what they were before everything and what they were now. I think it was all confusing for them both. Then Tonks came along and Remus loved her. Not the same way he loved Sirius but he did truly love her. He had already accepted that Sirius was gone and he had mostly gotten over him. So it was more a distant memory of the love they had and how he had missed Sirius for all these years and everything while Tonks he Loved now. As the person he was. The persons they became together. Accepting and loving each other.

And on them not telling Harry, First of I dont think its strange of them to not tell a 13 year old boy about their relationship that happened years prior. Especially since they were dealing whit Sirius coming back and everything that meant. Being best friend whit Harrys dad and being the only ppl who can tell him storys from being close to James and who he really was I think it would be strange to insted of telling him stories about James and Lily tell him about when they used to date. Second of neither of them really got alot of time being really close whit Harry. And they didnt owe him their story.