Who is the custodial parent in Marshall County (Plymouth), Indiana?

My sister is 17 (she lives with me) and had a baby at 16. We've already had a paternity test done (outside of the courts) as the boy didn't believe he was the father (which he is). She's still in school so the grandmother (father's mother) suggested watching the baby during the week while the kids go to school and we'd have the baby on the weekends. Over the summer we had the baby during the week and they had him on the weekends. My sister is a senior this year and she's decided she wants to have the baby during the week and put her son in daycare while she's at school. The grandmother (father's mother) says they don't want the child in daycare and that they should just keep things how they are. Doesn't my sister have custody of her son since they've never been to court?