I’ve had cats for 40 years now (crikey

We have three cats - Elsie, Reg and Fergal - who we adopted when they were tiny kittens almost four years ago. They’re all from the same litter, and all were neutered/spayed as early as our vet would do it. They have always got along; I wouldn’t say that they all adore each other but up until now after a bit of chasing each other around the house they settle together, and give each other ‘baths’. They go out during the day, coming and going, but they’re all indoors by the time darkness falls.
Recently however the play-fighting seems to have taken a dark turn, it’s almost as if the boys are ganging up on Elsie. She is not innocent of occasionally winding up one or other of her brothers, and when it’s one on one she can usually give as good as she gets. But recently both boys go after her at the same time, and whilst we are fairly sure they’re playing (we check frequently for evidence of physical injury) her yowls of protest are getting louder. I dread to think what language she’s using

I’ve been doing some research and it seems that neutered cats can retain some of their male-ness and Fergal in particular seems to be Adopting The Position with her so that would fit - except we’ve had them for nearly four years and this has never happened before. I also know that sometimes cats will pick on other cats who have medical issues but as far as we are aware Elsie is fine, she’s just half the size of her brothers. If it continues we will probably pop her to the vet to make absolutely sure but in the meantime I was wondering if anybody else had encountered this kind of behaviour seemingly out of the blue.
Any and all advice/suggestions welcome! And I apologise for the long first post!