I was told that they prolapse because their pelvic floor muscles are weak. It could be genetic, could be age related or down to internal pressure. Maybe the ewe is fat this year, or maybe she has particularly big twins. Next year she might be lighter and the lambs smaller so you could be lucky, but as you say, its not worth the stress. Good luck with her, I'd invest in a harness just in case. Be very carefull of her, if the prolapse if out for too long it'll dry out and turn dark red/black, once thats happened there's no going back and you'll need the vets to have her PTS.