you are doing the right thing for your girl! Adding a second dog will be an adjustment and there will be growls and maybe even some tough moments but in the long run having a sibling is the best socialization ... because growling at others out of jealousy is not nice
When we added our second dog - many years ago I followed the advice of my dog trainer and did not give in to the urge to pamper the new puppy more than my resident dog - I always made sure I pet, fed and loved on my resident dog first and then gave the puppy love. When my dog started showing jealousy I reprimanded him - a short low voice command ( no! ) - as you are the boss and can show affection to anyone you please ( and you pet your dog first !). Now - 13 years later the same two dogs are best friends and inseparable - it is touching how they check up on each other when one is sick- one of them was having a terrible time after surgery and my usually alpha dog let his sick brother eat first and spend hours on me me as if he knew that his friend needed the extra care ....