Yes, that's about right on the issue of Jesus' siblings (although the authorship of the letter of James is obviously disputed). As for the motive for the doctrine of Mary's perpetual virginity, that's a bit more complex and hard to pin down. The doctrine was common by the fourth century, together with the doctrine of Mary's virginity in parturition (according to which she gave birth to Jesus in a miraculous way which left her physically unmarked). These doctrines reflected the belief of many Christians that virginity had intrinsic value and should be preserved, together with the belief that Jesus' mother was the archetype for this endeavour. Some Christians disagreed, of course, such as Jovinian. He argued at the end of the fourth century that virginity was not preferable to being married, that all Christians are rewarded equally, and that Mary did not remain a virgin for ever more. The fact that Jovinian denied all of these doctrines indicates how closely connected they were in Christian moral theology at that time. There aren't really many claims to divinity in the Gospels at all. It is certainly hard to find any in the synoptics, at least any that are explicit, so it is a matter of disagreement whether, and to what degree, the authors of those Gospels think Jesus to be divine at all. The Gospel of John is another matter, but even there it is not necessarily clear precisely what is meant by the statements that ascribe divinity of some kind to Jesus. But however the belief in Jesus' divinity arose and became codified, I think the motive behind it is fairly clear. Christians believed that, in Christ, they would be saved. They also believed that salvation comes from God. Perhaps they experienced the new life they believed they were living in Christ as divine in origin. Taken together, these beliefs do not entail that Jesus was divine (he could have been just the conduit for a divine action), but it is easy to see how belief in Jesus' divinity could develop naturally out of them.