For this, you can leave the Case Management tab as "does not use cases." This controls the cases of the case type you've selected in your module.
The User Case will automatically be loaded into your form when you choose a form that has User Case setup.
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
Unfortunately, you cannot create a grandchild case in a form in a regular module. Without using advanced modules, the best thing you could do would be to open a child case and then go into another form in a module of the child case type and open the grandchild case there.