There's a LOT of really good advice already mentioned, but I want to most echo those of monksmommy. Perhaps, your partner is open to you operating and caring in the space of 'sister' or perhaps it might be something you mostly need to think of yourself as while playing Mommy. Either way, I think first and foremost it's all about embodying the space mentally. When I'm feeling especially not Mommyish, but need to take care of mine, I think of myself as sister or vicariously care for him in the ways I know I would like to when I'm feeling little.