Ugh. I'd hoped we were past using terms like "Irish twins" which a whole lot of people are going to find offensive.
I think 2.5 is young for many kids, so you have to sort of hang loose with your son. His sense of novelty may well wear off. 3 or 3.5 is pretty common, and many boys (it seems) are closer to 4. I think the 1.5 year old is fascinated by the novelty, and it may also wear off. Maybe she's developmentally ready, maybe not (probably not - but it would be great if she were the exception to the rule).
What they see and find fascinating (including stickers, toys and books) and what they can do developmentally because their brains actually feel the "bladder full" signal and it kicks in the whole sequence of "bladder full/go to bathroom/pull down pull-ups/sit on toilet/let go/tell Mom and Dad" are very different things. If you can keep in mind that this is not something you can really "train" a kid to do (despite the "potty training" term we all use), and if you can find a balance between being happy they tried without any disappointment that they didn't make it or stick with it, you'll be a happier family. It's easy to get excited with initial successes (and who wouldn't?), and I hope it works for you. But knowing you'll very likely be buying diapers for another year or more can help you stay realistic. The truth is, you don't know until they're done that they were truly ready.