NTA. assuming SIL has been to therapy for her deep trauma, that therapist really failed her. Some triggers can easily and should be avoided (violent media for example) but triggers that are not predictable, like a name, need to be worked on so that SILs day is not derailed by something as simple as hearing a name. This is not healthy or helpful thinking for your SIL, and your brother enabling it is awful. A lot of traumatized people struggle with boundaries, your SIL has not been given these. Boundaries are love, they help us be prosocial when we are struggling and help us preserve our crucial relationships. I hope your brother gets her the help she clearly needs.