DIL v DIL

Any advice please...
Son 1 has lived with girlfriend for 7 years.
Son 2 met girlfriend 2 years ago.
Couple 1 got engaged before couple 2 but couple 2 got married first.
Couple 1 are getting married next month and son 2 is best man.
Problem is this:
I had Dil 2 on phone in tears today as she thinks DiL 1 has an issue with her as shes been making comments about being jealous about them getting married first and only being together 2 years, about them having a cheap wedding and spending money thriftly (like my husband) and other comments putting couple 2 down. We have heard some of these comments but as they've been made in joking way and seemed to have been taken as such, have not said anything.
I am very easy going and dont get offended easily and I wonder if DiL2 is being a bit sensative (she's now talking about not going to the wedding) and DiL 1 is a bit thoughtless and likes being the centre of attention.
I dont want this to cause a rift, my H and stepmother didnt talk for 8 years and it was awful. We will tactfully say something if this happens in our presence and I dont know how to resolve this without making matters worse.
I would be very grateful to know your views and has anyone been in the same situation?