The issue with this is;
Jail breaking is already apart of the rules in multiple spots of the MOTD and states you must /advert jailbreak to break your family member out of jail. In a perfect RP scenario your family member would be transported to jail then you would attempt a jailbreak. If we create a rule allowing family members to assist their members in custody to help them escape, it opens up the doors for more FailRP on either A) The PD officer if multiple family members are present and have guns drawn or B) The family trying to assist their member, by spraying down a police vehicle as it's driving away and in RP those bullets could kill/harm your own member. This can also create situations where family members intentionally break a law to get arrested for the sole purpose of their family members rolling up and killing the PD involved created a grey area for cop bait.
In my experience if you are in handcuffs and the PD officer is present, family members can advert /assist to help you break free. So would that be something that you would want more clarification on in the MOTD then?