Identical twins have a closeness that I'd imagine does extend into their sexuality, depending on the degree to which the other is a part of their personal identity. I would imagine they can use each other as an expression of their own sexuality at times, live through the other's experiences. It must be alluring/ interesting to be able to see someone who looks identical to yourself getting off. I would not necessarily call such expressions incestuous in this case, even though the act sometimes may be, I'd call it more a form of mirrored masturbation.
As for twins on cam, they are usually fake, two non-sisters who look alike. I think I have seen these models the OP showed before, they are not twins or sisters. I know a model who is a twin, but her sister has never cammed, she just didn't have the interest or personality for it; I suspect for most twins this would be the case.
Sisters working on cam is very common; for example DannieRay (MFC) and MsAlanaGrant (MFC) are sisters, and have worked together occasionally. They don't do anything incestuous or against the rules, but for some working next to each other may be a step too close. I think it would be a pity to stop people camming from the same family if they can keep it within the boundary of no shared sexual contact.
Many sisters share a cam set up, one working while the other isn't, so where you catch them both on cam it is usually not for too long anyway.
I think in the case of real twins, I would allow some leeway with the rules on kissing and non-sexual touching because of what I described earlier, but that is up to the sites to determine. With fake twins, perhaps the same rules public may also be appropriate due to the fantasy content.