Complex sentence: His father, Santiago Jiménez Sr. was a pioneer of conjunto music, and his older brother Leonardo "Flaco" Jiménez is generally considered the greatest Tejano accordionist ever, certainly the most famous.
Split into simpler sentences: His father, Santiago Jiménez Sr. was a pioneer of conjunto music. His older brother Leonardo "Flaco" Jiménez is considered by many the greatest Tejano accordionist ever, certainly the most famous.