I think you must, neeny or they will feel they are only second-class grandchildren. If this relationship is The One for your son, then his partner's children will look on you as grandparents and you cannot treat them differently to the others.
If that is going to be financially difficult, that could be a problem. Will the others notice if their presents are not as expensive as usual to keep the total within reason? Probably not - children are not as worried about cost as about how interesting the present is.
What about other son? Will he and his partner notice? Will they care if they do notice? Can you speak to them about it before you buy? If they get on well with the stepchildren and look on them as more cousins for their own kids, perhaps it will not be the problem you anticipate. They may be including these new additions too.