Tennant or successor

I'm new here but anyway here goes.my mom and dad moved into there council home in December 1974.both names were on the rent book until my mom passed away in 2020.my dad's name was still on the book but he has since passed away.feb 2026.my two sisters lived there all there life but the council are saying that my sister can't apply to take over the house as when my mom passed away my dad was made a successor or something like that.whatever you call it.theyre saying you can only do it once or whatever. But I would've thought that my dad would've still been a Tennant.my dad was lost without my mom.he was 78 when mom passed so he wouldn't have known about successions or what not.council never told us about successions n that.just wondering where my sisters stand.its been our family home since 1974.my mom passed away in there.cou cil have told my sisters to start looking for properties