Hello there, welcome to COL!
Lots of dogs are like this, my cocker will barge in if I'm having a cuddle with my Weimaraner and the Weimaraner will sit on our heads if I'm having a cuddle with OH

It never becomes jealous and neither of them will guard us from the other so usually it's nothing to worry about.
To make the introduction go as well as possible, if it was me, I'd cool it a little with your current dog before getting your new pup so that less of your attention isn't a shock to her. Key to it is just carry on as normally as possible when the pup arrives. If there is any jealous behaviour ignore it and don't reward it by giving the jealous dog attention, just walk away. Try not to attribute them with human behaviour too much though because they do think and behave differently to us...
Very exciting for you!
Hannah x