Is this normal behaviour for a toddler ?

[mummypigsays]

Dry humping me, his sister and his toys, even the cushions?

Not normal, no, but kids do some odd things. If that is what he is really doing, my question would be what has he seen or been told by other children? It might just be a silly, fun thing he has discovered and he’ll have moved on to something else by next week. If he sees people get upset about it though, the novelty may take longer to wear off.

Some toddlers and young kids do this, yes. He’s figured out early that it feels good. Get him off his sister though ffs.

You can tell a toddler no? Especially if he’s dry humping his sister ffs

Tell him to stop. He’ll be laughed at when he goes to nursery.

Hmm. I'd recommend noone shares stories about their own children here.

Not so normal that they all do it, but normal enough I've known several kids that did (worked in day care/nursery settings).
How old is your daughter? It's never too early to talk about consent with them both "Please don't do that to me/sister, I/she doesn't like it". If he carries on then I'd use "stop, I/she said no" and move him away*

Otherwise though I'd be inclined to ignore or just distract him from doing it to inanimate objects rather than making that a "thing". Once a behaviour becomes a "thing", as pp said the novelty doesn't wear off the same!
By not acknowledging it it's a phase that will just pass relatively quickly in my, albeit anecdotal, experience.
I also think it's important not to shame him, which telling him off might do.

*Telling him for carrying on touching someone who doesn't want to be touched is different and isn't about the actual action being wrong if that makes sense, just like if that someone (or himself) doesn't want a hug/kiss/to be tickled/roughhouse play.

Good point (depressingly so)