I'm assuming in this case that you've loaded a the member case into your form, and that you can load the member case ID id into
/data/member_id
What you want to get is:
The count of all member cases which are children of this case's parent, for which the member case has the property mm_is_in_tb_treatment = 'yes'
For now we can create a new hidden value to store the parent household's case ID, since it'll make understanding the full query below easier.
New hidden value: /data/parent_id:
instance('casedb')/casedb/case[@case_id = /data/member_id]/index/parent
All cases which are children of that parent would be referenced as:
instance('casedb')/casedb/case[index/parent = /data/parent_id]
You can now add your new filter directly to the end of that statement and introduce the count
count(instance('casedb')/casedb/case[index/parent = /data/parent_id][mm_is_in_tb_treatment = 'yes']
If you wanted, you could further specify that you (for safety) only want to include children that are member cases and which are open. You can also collapse back the parent ID reference if you want a one-line expression:
count(instance('casedb')/casedb/case[@case_type='member'][@status='open'][index/parent = instance('casedb')/casedb/case[@case_id = /data/member_id]/index/parent][mm_is_in_tb_treatment = 'yes']
Best Answer
This trigger system only supports one expression.
A way to incorporate multiple expressions or calculations into the SMS trigger is to store the outcome of the expression in a hidden value in a corresponding form, and then save that outcome as a case property. This single outcome saved as a property can then be the one property that triggers the SMS reminder.