Housing benefit stopped

My sister is due to receive her 1st UC payment on 18/06/2025. She was on IR ESA and support group and receiving housing benefit. All this has now stopped as she was informed it would 2 wks after she submitted her UC claim. However this wks rent has not been paid and she is now in arrears by the full amount of rent . She is in sheltered housing in social housing. She received a letter from the housing benefit saying she has to re apply for housing benefit. I am in the process of doing this on line for her but I can’t submit this claim as it wants to know how much UC benefit she gets, my sister doesn’t know how much her UC benefit will be. I phoned UC to ask them they can’t tell her yet what it will be. So she will be in 3 wks in rent arrears until her first payment on 18/06/2025. Is this correct.