Yes, you can!  You can use the 'Batch Check In' feature to select multiple participants (in this case, siblings) at the same time.  Simply click on 'Batch Check In' at the bottom of the screen, then select the participants you wish to check in and follow the prompts!  Please note that this is available for ADMINS only currently. If a parent is checking siblings in, they will need to do it for each child or you can use the option of Batch Check In and choose "Other" and add their name instead.