The simple answer is yes, it is going to change the way your kids interact with their dad and his new wife.  Change always causes some stress.

The important thing to do now before the baby arrives is to have a family meeting and talk through all the concerns that you have.  Spend some time making a list of what you think will cause issues and get your kids to do the same.  At the meeting, talk through it and come up with solutions to each potential problem now, before the baby arrives.

You never know, they might take to a new sibling very well, but it is better to be prepared just in case.