I do think most twin mums know of this potential scenario, and I'm so sorry to hear it's happened to your sister. It is more on the extreme end of what can happen.
I'm very clear that mine are considered two separate people by others. Shared cards are not ok etc.
However what's best in an ideal world for the child isn't always what the parent can do on a day to day practical sense.
I think, yes, twins can absolutely provide their real lived experience of how certain things felt. But you can't understand what it's like to raise twins. Whilst I feel utterly privileged to have them, there are days where you just have to do what's easiest to get through. And I say this as a mother to singletons too. Who are a walk in the park in comparison.
As parents, we do our best. Well, most of us. And if it means you can actually get out of the house with difficult twin toddlers because it's the only hour that day you can get to the supermarket, then yes, they both get put in the same outfit for sake of ease. Because we'll be stopped by twenty people as we're trying to go round, regardless of what you're wearing, because "oooohhhh twins!" and that's going to make it all time pressured enough.
We definitely know our twins need their individuality. It's how most (not all) thrive. We also need to get through a form of parenting that simply can't be described, only experienced.