I also try to avoid calling them anything when talking to them. My sister is close to my in-laws and she calls them Mom and Dad, yet I'm a little uncomfortable with it. (My in-laws call my sister their "adopted daughter". When I became a part of their family she did too. ) Really though it depends on who I'm talking to. When talking to my husband I call them "your parents". If I'm talking to Nathan I call them Grandma and Grandpa. If I'm talking to my sister I refer to them as Mom Blake and Dad Blake. If I'm talking to someone who knows them personally I refer to them by their first names. If I'm talking to someone who doesn't know them I call them "Justin's parents" or "my in-laws".