Vasiliy, Windchill creates the wt.part.WTPartUsageLinkobject when you assign the child WTpart to the parent WTPart. It is a master to reference link, so master of Parent Part is attached to the each version of child WtPart.
To get the Parent information form the WTPart you can use nevigate APIs. Pass in the child Part and get the Parent information.
ParentWTPPart <----->wt.part.WTPartUsageLink<-------->ChildWTPart.
PersistenceHelper.manager.navigate(). for more information refer the Windchill API Doc.