Answer
According to the Shari’ah, an illegitimate child will be attributed only to the mother. (Shaami vol.23 pg.489; HM Saeed). Therefore, the child will inherit from the mother only. The biological father may make a bequest (maximum of one third of the estate after funeral expenses and debts) in favour of his illegitimate child. From: Al Mahmood 13 Q & A from the Darul Ifta Mufti Ebrahim Desai