Re: GURPS 4e Rules Chat
The short answer to all those questions; however much your GM will allow. :)
Okay, that’s probably a cop-out answer. As far as attributes go, you should probably determine a base for both of them and not vary them by much more than a point or two. It wouldn’t be realistic for one twin to have an 8 ST and the other an 18...but, if you decide on a base of 12, and give one twin an 11 and the other one a 14 it’s believable. There could be extreme possibilities, like if one’s a bodybuilder and the other a shiftless lay-about, but beyond that the attributes should be close. Even IQ could vary by a point or two without being unbelievable.
Physical advantage can vary, and some can’t, for obvious reasons. Appearance, obviously, should be the same. Unless one has had some permanent scarring or such. Or, in a modern or later setting, plastic surgery to improve their looks. But, again, those are extreme circumstances -- generally, identical twins should look, well, identical. As for other physical advantages/disadvantages? As a rule of thumb, I’d say if it can be picked up after character creation, it would be something that could vary between them. Most GM’s would probably allow you to buy Fit later on, but few would allow Ambidextrous, for example.
Metal ads and disads? Again, I’d say if it can be picked up after character creation, go for it. I dunno about Kleptomania; but it can be cured, so if one twin has it and not the other, you could probably get away with saying the latter got it cured with therapy. Things like phobia’s, obviously can vary; and I’d say character traits (impulsive, honest, stubborn, etc.) can definitely vary.