You are correct and the Saud will call the other Sunni to help with assistance. As they already have done. The Saud is amassing troops as we speak although it appears defensively. The Shia Rebels have driven Hadi from his home and are advancing. If they push for Aden. The Saud and others will jump in.
(Reuters) - Saudi Arabia is moving heavy military equipment including artillery to areas near its border with Yemen, U.S. officials said on Tuesday, raising the risk that the Middle East’s top oil power will be drawn into the worsening Yemeni conflict.
The buildup follows a southward advance by Iranian-backed Houthi Shi'ite militants who took control of the capital Sanaa in September and seized the central city of Taiz at the weekend as they move closer to the new southern base of U.S.-supported President Abd-Rabbu Mansour Hadi.
Riyadh hosted top-level talks with Gulf Arab neighbours on Saturday that backed Hadi as Yemen's legitimate president and offered "all efforts" to preserve the country's stability.....snip~
Exclusive - Saudi Arabia building up military near Yemen border: U.S. officials | Reuters