Credit checks/ Mortgage Application

My sister has young adult children both of whom are planning to purchase their own properties with their partners.

I told her that there might be a problem as both are still (ever since university days) using their family home as their main address whilst residing elsewhere. One in same city as family home (in large block of flats where post often doesn’t arrive ) the other in a different city (with address changes due to job).

My sister is now concerned that she mistakenly advised them to stick with the family home to ensure credit cards and confidential post are always safely received. Same applies to the electoral roll.

They are both paying their respective share of Council Tax in their flats, though unsure whether the bills are in their names or their partners’.

I wonder if this will cause difficulties (by providing potentially conflicting information), for credit checks when they apply for mortgages?

Given their home address remains the most stable for the reasons stated, how best should this be managed?

Thanks in advance