Congratulations! Perhaps you should read the first of today's threads, so you don't let yourself be shanghaied into more child-care than you are up to!
It's all too easy to offer and all to hard to stop again.
If the seven year old is jealous, you and his parents will just have to tell him what a good big brother he is. Babies are boring for a seven year old boy, so maybe you can find some things he and you can do together "Now he is a big boy!"
He may never be jealous at all, as he is big enough to understand to a certain extent that the little one needs mummy's time and attention.
Cross the bridge if and when you come to it. Don't spoil the months of looking forward by worrying about something that might never happen.