Nah, the immaculate conception is rather irrelevant to the issue. If Mary's holy enough to bear Jesus, that means nothing about her being human enough to bear more kids; twins or not.the resurrection is only half the story. the first half has to do with an immaculate conception (can't believe only one person made mention of this very important aspect). the other siblings are completely different in that they aren't divinely created. IF twins pop out, both have to be considered divine. how they create the ministry becomes another tale. Overall, though, if God wanted twins, he would have created twins, so the concept is ASB from the start.
Plenty of Christian groups have gotten away with not caring about the exact holiness of Mary, but none have gone around the resurrection. Because that victory over Death is the whole point.
And, of course, plenty of Christian groups disagree about Jesus' divinity. Was he, as in the Chalcedonian view, both God and Man (or the Monophysite mix of God and Man)? Or was he God-only with human dressings (as the Docetists have it)? A human who was so Holy that God effectively adopted Him (the Adoptionist position, though they probably disagree on if the original-Jesus was Holy enough, or simply a suitable vessel that became so Holy thanks to God picking him)? And apologies to anyone whose Christology I've butchered here.