Yes, there are many Forsytes. There is a detailed family tree in the Oxford edition, but if you read it now, you will encounter spoilers. Perhaps I can summarize for you.

Old Jolyon (who I like a lot too) is really Jolyon III. The generation before Old Jolyon's is mentioned but not relevant to the story. There are 10 siblings. The story is about this generation, noted here in birth order: Ann, Old Jolyon, James, Swithin, Roger, Julie, Hester, Nicholas, Timothy, and Susan. All 10 siblings do not get equal play -- we will read much more about Old Jolyon, James, Swithin, and Timothy. The sisters Ann, Julie and Hester are mentioned much more than Susan; but they don't feel like key characters to me, only background in describing the family and society.

Only 5 of these 10 siblings have children. From this generation, the most important characters are the children of Old Jolyon (Young Jolyon) and James (Soames and Winifred). They are all mentioned in this week's chapters.

The children of these 3 characters, or grandchildren if you will, also play a large role, but with the exception of June (daugher of Young Jolyon), we haven't read about them yet. I don't remember exactly when the rest of the children start to play a major role (all the books which I read 1.5 years ago are a blur now and I don't want to read ahead and accidentally post spoilers), but you will know when you see their names.