T is right - SLOW is the key. Although the new girl is older, the younger has "owned" you for three years. She's going to feel displaced. Make it easy for her to accept the newcomer and respect her "space."
I had an Irish Setter (male) for several years. Then brought in a female (rescue dog) Setter. I was lucky. After a "discussion" about sleeping arrangements, they worked things out. Several years later we had a couple of litters of Setter pups. I don't have any magic - these two did well by themselves. But the key to me back then was to keep it slow, keep them apart for awhile, let them spend some time together, then apart. And while helping the new dog get acquainted with the family, never let the "old" dog feel replaced. It took a lot of work, but ultimately was well worth it.