I don't know which piping code you're using, but in B31.3, reducers have a SIF = 1.

And without routing, there's not a whole lot of specific detail we can offer.

First line of thought:
Is the PSV to an open or closed system?
Which method did you use to calculate relief loads?
Are you over-constraining your pipe based on over-exaggerated relief loads, thus not providing flexibility for thermal?

Second line of thought:
Is the PSV small?
Some piping specifications only consider pressure when determining pipe thickness, which means small diameter piping become extra thin.
This results in you trying to balance loads on piping that's not much thicker than an aluminum can.

Third line of thought:
Are your displacements from your main line causing issues in your smaller branch line? Restrain the main line.

Fourth line of thought:
How stiff are your supports? CAESAR probably thinks it's 10e12 lb/in (or whatever it uses in SI notation). That's approximately the load to move one Pyramid of Giza 1/2". Perhaps you can estimate a more realistic stiffness.