Our youngest is such a trooper, and is always down to play with her brother's friends! Sine they went to the same daycare for a year, she actually knows a lot of his old daycare friends really well.
But now that she's going to her own preschool, we've been making a bigger effort to give her opportunities to have playdates with her own friends. It's harder since she wants to play with her brother's older friends more than our eldest wants to play with her younger friends! But we have some friends with two kids with a similar age gap, and that's made it a bit easier logistically. Still, it's something we're working on - just to be fair to her!
Does your youngest play more with their own friends, or with their older sibling's friends?