The unpaired electrons in Al and Si are present in 3p orbital.

Nuclear charge is defined as the net positive charge experienced by an electron in a multi-electron atom.

The higher the atomic number, the higher is the nuclear charge. Silicon has 14 protons while aluminium has 13 protons. Hence, silicon has a larger nuclear charge of $(+14)$ than aluminium, which has a nuclear charge of $(+13)$. Thus, the electrons in the $3 p$ orbital of silicon will experience a more effective nuclear charge than aluminium.