OP I think your issues with your SIL are bigger than the wedding. Part of siblings partnering and marrying often means a re-structuring if family networks and celebrations. It can be hurtful or maybe even bewildering, I don't think it's ever worth cutting ties. I think you will regret cutting your brother out for this if that's what you do. Keep your dignity, stiff upper lip and either go or politely explain your reasons for being unable to attend.