unless the family members repeat group is being used to create cases of a different type “Member” for example, you won’t be able to use this feature to get individual members to pick from.
in order to accomplish that, check out this page
and then, when the child cases are created, you can change the query you had for families and change the case type to “members”. what will happen if you leave things like this though, is that you’ll get a list of all the members for all the families (likely huge). in order to make things easier, you might want to add a question that asks something like first letter of the first name, or to select the family before you select the member (another multi-select with cases but this type it’s families not members), and add a clause to the members query to filter by that question.
if you are planning on tracking the members themselves and what happens to them over time, your form structures would have to change, as you would have to select the member case before you can load the form so that you can update that specific case.