Nihilus wasn't stronger than Kreia. It's just that his specific "power" was to suck the life out of Force sensitives, weakening them. On Malachor, where the DS is especially strong, that was too much of a combination for Kreia to handle. However, Nihilus wouldn't have been able to cast Kreia out without Sion's "muscle". If you'll notice in the cutscene, Nihilus is the one that drains the Force from Kreia and blocks her Force useage, but it's Sion who actually physically "finishes her off".
I'm not sure why you insist on diminishing Sion's part in the story, despite the fact that he was *needed* for the betrayal and Kreia's motivations for training the Exile to make sense. You keep saying they could have just made Nihilus take Sion's roles, but then you're changing what the story is about.