"My sister recently married and they are discussing wether her being married to an outsider will stumble others and if they need to reprove or disfellowship her for it."
The question becomes: Who will be getting stumbled? If the congregation punishes her, how will every "outsider" who hears about the situation feel? If they're looking to completely alienate the husband and his family, and any of her non-JW family, go ahead and go JC crazy! That will drive more people away from the bOrg! That happened with more than one of the kids I grew up with. They left and never looked back.
Or if they think they're going to make an example of her by punishing her in front of all the children in the congregation, is that really going to work? They've been DFing people like crazy for decades and they are still losing about 2/3 of their kids. (Higher in my congregation.)