RCN head Peter Carter said he did not want relatives performing tasks nurses were employed to carry out, but that there were "real benefits" for patients when family members helped with care.
The Department of Health said family help needed to be alongside NHS care.
But patients' groups warned such a move could be "the tip of the iceberg".
Dr Carter, the RCN's general secretary, said the college was not suggesting families be compelled to carry out any tasks.
"We know that there are real benefits for patients where relatives can get involved in care, if that is what both the patient and family want," he said.
"We know from areas such as children's care that having familiar people involved at mealtimes for example can make hospital stays in particular less stressful for all concerned.
"What we would like to see is flexibility to allow relatives to help make patients comfortable, such as extending visiting times."
Department of Health chief nursing officer Christine Beasley praised the "amazing work" work of carers and relatives and welcomed their help but added: "This must be in addition to NHS care, not instead of it.
"Nurses should spend their time caring for patients and it is important to look at the way wards are run to help ensure this happens."
"I expect all hospitals to ensure that they are providing safe, high quality nursing care because this must be at the heart of the NHS."
But Patients Association chief executive Katherine Murphy said patient care - including helping with feeding and taking patients to the toilet - should be carried out by nurses.
"It is just the tip of the iceberg," she said. "Where will we draw the line?"
She added that some patients would not have families nearby or with the time to help out.
If there were not enough nurses to provide the care, then more nurses needed to be employed, she said.
Earlier this week, Dr Carter said the NHS had become too reliant on healthcare assistants who often end up doing more than the basic tasks they were employed to do.
He recommended better training and regulation of health care assistants.
Hospital visiting times should be extended so patients' relatives can become more involved in their care, the Royal College of Nursing has said.