It is soooo hard to advise you what to do. you have heard what it is to go to a parents wedding. I agree with farrockgrandma, about the 3rd.4th.5th………..time. look at the bright side if they go, then you have a whole evening to pesach clean without your little helpers around.
Ignore the outside family. Just think of the reprcussions if they do not come. Will “She” {their “stepmother”) hold it against them at a later date? It is nice when there is a “working relationship” with the New Wife.
Just remember to have extra Kavana with “Pokeach Ivrim”, and also “Ata Chonen Ladam Daas.”
May HKBH give you extra Siyata Dishmaya to overcome this nisayon.