My uncle died without a will. He has no spouse or children. He has 5 siblings (one deceased who is my mother). Are we able to share in his inheritance with the 4 living siblings (me and my sister)?

No. Under CA law, his siblings would share in the estate. However, the only way you would share if your mother's share went into her estate. And since she predeceased your uncle, she does not get a share.

Intestate Succession in California | Nolo.com