Nothing new, always have been but I chose to be with their son regardless as he is obviously isn't, and openly disagrees with their views. He has not spoken to his dad in years because of it. I have never met his dad or entered their house - this is fine by me, I don't need them as I have my (supportive) family, who have also supported DP in some very challenging times.
When our second son was born (nearly a year ago) MIL decided she wanted in, and wasn't going to follow the family's 'principles', she came and stayed for a few days she was a pain in the arse but it finally started to feel like things might be going in the right direction.
We allowed the DC to visit MIL when FIL was not there and they had a great time, DP is obviously very excited that his family might be learning.
Fast forward to yesterday, wedding invites have been sent out. MIL phones me saying that she will not be attending as FIL will not allow DP's youngest siblings to attend as they do not want them influenced by a 'mixed heritage relationship' and 'their principles still stand' and that they had decided they didn't want their 2 youngest children to come into contact with me or my family as they may 'also be swayed' like DP.
I told her that it's tragic that they can't put aside their prejudices for their son and grandchildren but that is their choice, however they would not be in contact with my children any longer as my children are mixed race and I do not want them exposed to such racism (whether mixed race or not)
So now, my children have no grandparents on that side and won't be seeing their DU's for a long time and NONE of DP's family are coming to our wedding. His elder sister is financially dependent on FIL so she is not coming incase he cuts her off (he's done it before)
I know it's not my fault it's theirs but I feel so guilty. 
DP is positive that he wants the day to go ahead and that is it their loss but he is obviously very sad too. Was I right to cut their contact with the DC?