Devi usare addRule, specificando l'id a cui vuoi ancorare.
public void addRule (int verb, int anchor)
Since: API Level 1
Adds a layout rule to be interpreted by the RelativeLayout. Use this for verbs that take a target, such as a sibling (ALIGN_RIGHT) or a boolean value (VISIBLE).
verb One of the verbs defined by RelativeLayout, such as ALIGN_WITH_PARENT_LEFT.
anchor The id of another view to use as an anchor, or a boolean value(represented as TRUE) for true or 0 for false). For verbs that don't refer to another sibling (for example, ALIGN_WITH_PARENT_BOTTOM) just use -1.