There are such families in which one son and his wife, kids, grandkids are Sikhs and another son and his family are Hindus.
As much as I have observed it is such families who are likely to call Sikhs Hindus or call themselves also Hindus and perform Hindu rituals as well. Due to such families Sikhs are called Hindus by many Hindus also.
Where did such families come from? Such families are most confusing. Some of them have say, an Amritdhari sister and jija but cousin will be a Hindu with cut bair, Hindu name and all. Or mother in law, father in law will be Hindu but only husband will be Sik