While no US based carrier has this feature, several foreign frequent flyer programs have this with a low annual fee (about $30 or so).

Off the top of my head would be JAL Family Club and ANA Family Account

Since both JL and NH are oneworld and Star Alliance members respectively, you can bank yours and your family's AA or UA miles in these programs and share the miles amongst each other if this works out better for you.

Mind you that while there's no restriction of US based members in joining these family account programs, JL and NH are Japanese frequent flyer programs so most of the promos will be geared toward residents of Japan, much as our AAdvantage promos and bonus offers are targeted for US residents. So I suggest that you think this through if what you want will outweigh the benefits of just creditting your AA miles to AAdvantage over JAL Mileage Bank.