I have 4 dds. The first three are all 1.5 years apart from each other and the youngest of 4 is 2.5 younger than the youngest of the first three. It doesn't help that the youngest started school late. Anyways, dd 1, 2 and 3 are very close and do a lot together while dd4 is always left out. While I am very happy they get to have this bond together it can feel a bit mean seeing dd4 always on her own.
I do feel like personality does come into it and there is not much I can do about it without being unfair to the older 3: dd3 is mature for her age and would not have much in common with dd4. For example, when we go outdoors as a family DH and I would relax in the sun while the kids hangout and play. dd1, 2, and 3 would talk endlessly to each other and have a great time filming stuff on their phones together while dd4 is always on her own.
dd4 is a bit young for her age and can be immature but she is very sweet and loves her older sisters so much. She is a bit upset how her sisters would all prefer each other over her. I have taught her to ask nicely if she could join them but my older three would always turn her down nicely, which isn't wrong so I don't know how to interfere.
By the way they are all teenagers and dd4 is a tween.