Alright – I'll bite and give you MOO.
1) Is this a credible scene? Sure – why not? Unfortunately these things do happen in society. wrong place / wrong time / wrong clothes
2) In said situation would older sister offer to protect younger? Most definitely. Is nature to protect our siblings, and particularly younger ones
3) Younger sister / housewife would offer the same for the same reasons – in protection for her sibling, but the circumstances would have to warrant it – most likely setup? A deterioration of her physical condition that would prompt little sister to intervene in order to feel that this was necessary to “save” her . . . i.e. after fourth man pounds into big sisters rectum, blood is now running down her legs and pooling on the ground and she about to pass out and little sister is positive if 5th and 6th man continue, sister will die.
4) Aftermath? First physical damage: Varied according to position and if rapes occurred vaginally or rectally, but standard is going to be torn clothing, possibly ripped out hair, choked / gagged = bruising, busted mouth, nose, bites are very likely, broken or missing shoes, possibly broken teeth, expect bleeding possibly profuse if rectal rape, and other injuries according to position i.e. on pavement torn skin off back, buttocks, etc. / against metal fence, possible puncture wounds and tears from loose pieces of wire, etc. / against wood, splinters, etc. You get the idea. It would be difficult to walk and they would be in obvious pain, expect stumbling, vomiting, etc. Emotional? Bouts of hysteria and shock that could last days or even weeks, possibly longer. Immediately after? Sisters would try to comfort one another but primary goal would be to get the hell away from there by aiding one another and getting somewhere safe, then to clean themselves of all evidence pertaining to this – burn clothes, scrub their skin raw, drink alcohol out of desperation to get taste of nasty man from tongue, etc. (I am assuming the police will not be involved?)
Does viewing or participating in gang rape equate little sister turning into slut? No – not quickly or slowly. But a very reasonable untreated emotional response could easily be perceived as one of two extremes - the “nun” approach, no man ever looks or touches me again, I’ll run screaming from the man who said hello in the grocery store - or the “slut” approach, I’m worthless now so why not? Go ahead and fuck me. You are obviously interested in the latter effect.
Person observations? There would be very few gangs of rapist thugs who would take liberties with one female and simply allow the second to watch unless there was something extremely unappealing physically about her . . . and I would see perceived innocence as simply a further turn on. Possibly one or two members might have some semblance of pity and waylay true rape for forced hand / mouth, but again, very unlikely. And you're gonna need a reason why these thugs don't kill the girls afterward.
Hope it works out for ya! Best of Luck! ~E~