Am I permitted to prevent my stepmother—after her divorce from my father—from marrying another man, given that I consider her my second mother and have siblings from her?

You do not have the right to prevent your father's divorced wife from marrying after her iddah (waiting period) has expired, on the pretext of jealousy. If you do so, you are overstepping the bounds of Allah and oppressing her. The Sharia prohibits a man from preventing his daughter from marrying a suitable partner, so how much more so for one who prevents a woman over whom he has no guardianship? Therefore, fear Allah and do not prevent the woman without a just cause.