I think we’re using “sibling eviction” to refer to evicting a non-conflicting tx that spends a different output from a common parent. (I guess it would be cousin eviction if it was a non-conflicting tx that shares an ancestor, or was in the same cluster?)
RBF without sibling eviction will only replace txs when there’s a conflict; if there’s no conflict but other rules are broken (too many ancestors/descendants, too many txs in the cluster), then RBF without sibling eviction would just fail, no matter how much higher the fee was. At least, that’s how I understand the terminology?