The bottom line here is that (except for the French code CODETI and the new EN-13480) no piping Codes address a "wye" fitting. Therefore, since there are no Code guidelines you are left to make your own engineering judgement call.

Two options are:
1) Utilize FEA software as mentioned above
2) Contact the fitting vendor and ask them for the SIF (maybe they can provide this information).

What do CODETI and EN-13480 say about a "wye" fitting (actually a spherical wye)?

i = 0.9 / h^.667
where h = 2*en/dm
en = wall thickness of the sphere
dm = diameter of the sphere

Before you use this information, please look it up and verify it yourself.