On the Android app if you want to add a parent or spouse, you first need to go to the parent or spouse's memorial, click on the three dots and select Pin.
To Remove that pin you can do the same, click on the three dots and select Remove Pin.
Once you've pinned the memorial go to the child or spouse where you want to add the relative and click the three dots and select Edit/Suggest Edit.
Select the relationship, Mother, Father or Spouse.
This will bring you to a list of all the memorials that you have pinned. Select the one with the correct relationship and then Save.