I am not trying to be insensitive here but I am going to have to tell it like it is:
Your mom and dad need to say NO to their demands for jahaz.
Buying these things now means for the rest of they will keep getting emotionally blackmailed so their daughter doesn’t end up divorced. You’re not helping your sister if you guys allow this to continue. You’re sending her into a family that will never value her or respect her - just the stuff she bought with her. Well what happens once that stuff is gone, finished and used up? There will be more demands…more angry requests…more subtle hints.
Say NO and if the husband bolts - let him. Its better than him bolting after children.