Yousaf's parents-in-law live in Scotland but were visiting relatives in Gaza when Hamas militants poured into Israel and killed 1,300 people last weekend.
"There is no doubt that collective punishment is a breach of international law," he said.
British Prime Minister Rishi Sunak told parliament on Monday that Israel must defend itself in line with international humanitarian law.
Diplomatic efforts are continuing to get aid into the enclave as food, fuel and water run short.
He added that his brother-in-law, who works as a doctor, is having to make decisions about who should be treated, while the hospitals in Gaza run out of body bags.
Persons:
Humza Yousaf, Yousaf's, Israel, Yousaf, Elizabeth, Maged, Nakla, Gerhard, Rishi Sunak, Andrew MacAskill, Kate Holton, Andrew Cawthorne
Organizations:
Reuters, Scottish National Party, REUTERS, London ., Thomson
Locations:
Gaza, ABERDEEN, Scotland, Israel, Aberdeen, Britain, British, London, London . British, Dundee, Palestinian, Scottish, Glasgow, Pakistan, Kenya