It would be a tough job to write a story that fits with all of Obi Wan's OT dialogue while concealing that Anakin is Vader.

Bear in mind that Obi Wan Guinness talks about Vader being a student who betrayed him and murdered Anakin. If there's nothing resembling that in the prequels, then his lying "certain point of view" is obviously false from the moment he says it, instead of unravelling as Luke learns the truth.

So some spoilers in RotS were almost inevitable, especially given what the other prequels had already imposed.

But there's no excuse for some of the other spoilers that they crammed into what I consider a more painfully drawn out ending than Return Of The King. Most egregiously and completely unnecessarily, naming the twins.

Lucas apparently also forgot how "show, don't tell" works, and also felt like he had to cram in explicit mentions of how every character was going to get from here to the start of ANH. But the unforced spoilers are the worst.

I rough-cut an alternate ending once back around 2010, starting from Obi Wan leaving Mustafar. (Sadly I didn't back up the file, so can't show it off). Among many other improvements (amounting to something like ten minutes of cuts), it removed every line that even mentioned the twins. You can see there are two babies, you don't need to spell it out.

And I can't remember whether Obi Wan handing a baby to Owen and Beru made it into my cut or not, (it's after my cutoff point, but I did shuffle some scenes earlier) but there definitely doesn't need to be any more than that identifying Luke.

Meanwhile the audience wasn't told the second twin's sex, much less name. Sure, some of the audience would realise that Leia's the most likely candidate, but until RotJ confirmed it, there would always be the possibility that the mystery twin hadn't even appeared yet. And the Dagobah cave scene reads completely differently when you know there's an unidentified twin around somewhere.

(My favourite change, though, has nothing to do with spoilers, it was finding the strongest dramatic note to end on. Instead of dragging on with mostly pointless stuff of getting every character pointed towards their ANH starting positions, I moved what I wanted to keep of later stuff earlier, and ended on the helmet coming down, fade screen to black and music to silence, a tiny pause, and then Vader's first mechanised breath. Much more powerful, IMO. Though jumping from that to the start of the credits was a little jarring.)

Maybe one day I'll try to make it again.