health insurance for parents-in-law living elsewhere in the EU

Dear all,
I want to get my parents-in-law insured with the mutuelle in Belgium. They live in an EU country.
Apparently they have to get an identity card issued in my commune here in BXL but I can't find the exact information for this issue.
Has anyone done this before or knows about it? Any advice or links would be valuable.
TIA and cheers,
M