I’ve just finished the first three books in Jill Shalvis’s Lucky Harbor series, which focuses on three sisters opening an inn left to them by their mother in the small town of Lucky Harbor, Washington. Romance series seem to frequently focus on siblings, with each sibling having their own book. One of my first experiences with this common phenomenon was the Bridgerton series by Julia Quinn, which spanned 8(!) books. These kinds of series are a great way to introduce the future heroines and heroes of romances, and leave the reader anticipating every new title. Although you would think that it would be expensive for the family to have so many weddings so close together.
Jennifer’s List of Series Featuring Siblings
The Huxtable Quintet by Mary Balogh
1. First Comes Marriage 
2. Then Comes Seduction 
3. At Last Comes Love 
4. Seducing an Angel 
5. A Secret Affair (technically about a cousin, but close enough)