Hi Carol,
Not sure you will be able to gain much information from the uniform as it is so basic other than the 1908 pattern webbing belt, you can look through hundreds of WW1 photographs and you will see this belt being worn as in your photograph, so it is not unusual. What is unusual is that there is no shoulder titles or other distinguishing insignia on the tunic, the buttons appear to be standard brass button.
Now with regards to siblings dressing up in one of the others uniform you can discount that in this case, as 292106 John Campbell KIA 23 April 1917 in Egypt was in the Cameronians ( Scottish Rifles) and his buttons on the tunic would be Black as in all rifle regiments.
All I can say is that this photograph was taken prior to 1917, but by looking for Johns service record you may be able to help date the period, for example on his MIC there will give a date when he entered the theatre of war and was away from home.
Kind regards
Hiram