Sequence number semantics
If you're creating a version 2 transaction with the disable flag not set, then by definition the whole nSequence value will be less than 0xFFFFFFFE (because it will be at most 0x7FFFFFFF).

This implies that a transaction with an active relative locktime will always be replacable according to BIP125.

This is intentional. Nonreplacable transactions attempt to make use of the existing network policy rules to favor the first broadcasted version of a transaction. A transaction that will only become valid at a future point in time cannot make use of that, as it can't be broadcast until it becomes valid.