Family members can do several things to support the person or people with OCD in their family.
First, they can become involved in the therapy. Having family members encourage the people with OCD to keep up their exposure therapy or take their medication can help them to challenge their OCD and not give in to it.
Family members can also offer encouragement to people with OCD while they are doing their exposures. Family members can help move therapy along, because they may be able to tell the therapist what OCD behaviors they see being performed that the people with OCD may not even notice or may be too embarrassed to discuss.
This can help therapists to be sure that they are designing exposures to fit all of the difficulties the people with OCD are experiencing.
Most of all, family members can be role models for their family members with OCD. They can do the exposures, initially, with the people with OCD to show that the feared consequences do not happen.
Of course, you will want them to do the exposures on their own eventually, but modeling can be of great benefit at the start of the therapy. Family members can also work together to be sure that no one in the family is accommodating the OCD.
Sometimes people with OCD will try to manipulate someone in their family into helping them with a ritual or giving them reassurance. If family members can work together, then they can assist each other to not give in to the OCD demands, but to present a unified front and work together to challenge it.
Remember that it is the OCD that is being challenged. Of course you love the family member with the disorder, but that does not mean you have to love the OCD.
Remind family members with OCD of this, to let them know that in order for you to be helpful, you can no longer help them do rituals but will support them in fighting through their difficulties.