If the OP keeps it secret, that will be a green light for Uncle Inappropriate to do / say more. It will become, as far as he is concerned, 'our little secret.' What's the next stage ? It's ok, I hear you say, he said nothing will ever come of it. Really ?
I agree regarding his arrogance. There's also an element of gauntlet throwing in the phrase, 'think about it.' I'm concerned whether or not the uncle had to work up the nerve to say it. He obviously doesn't care what impact it will have on your, your husband, the wider family etc.
I'm not saying the uncle is a stalker, but a lot of stalkers genuinely believe that they will be together with their 'prey' one day. Probably in the afterlife. Anyway, moving swiftly on.
No offence to the OP but is she just kind of at hand because they live in the same street, belong to the same family ? It's just to easy for a 15 year old girl to have a crush on her seventeen year old sister's boyfriend. However, if said bloke was available, he might not ever come to her notice.
I'm also a bit miffed that the uncle thinks he can say this totally inappropriate thing with the caveat, 'I know nothing will come of it'. Um, so why say it ? It smacks of, 'the offer's there if you want it.'
I can't help but wonder who else he's been 'mooning' over.
I'd ask if the uncle's social boundaries are so totally skewed that he thought this was ok and there would be no comebacks. It rather smacks of attention seeking too.
What's the uncle's defence going to be if it all comes out ? 'I only said' or 'she tempted me' or 'she led me on' ?
Maybe in the uncle's warped fantasy mind the OP works from home so she can be available to him. Which weaves into what a pp remarked on did he take it as a huge hint that they moved into the same street.