There is a field in inSMS schema call linkedSmsId. you can use this field to set broadlogId. Since it's not auto populated so for this you can create one tech Workflow which links incoming SMS with originating broadlogs.
This workflow runs as delta and picks all data from inSMS as first query and then you can add enrichment with rcpbroadlogs join conditions based on extAccountid and messagetype. This will be 1:N, to restrict the output to 1 use data collected as Limit the count in properties and filter based on address=(origin) and sort by eventDate desc.
After enrichment you will have latets records for that origin from rcpbroadlog and pass broadlogsID to next activity which will update inSMS table and column linkedSmsId.
But carefull, there is no "easy" way to be 100% sure for a response to be linked to the outgoing SMS as this depends from the SMS provider arbitrary rules. That's not coming from Adobe Campaign it self but about the amount of phone number to reply to a SMS in order to identify the answer.
=> usually it is based on a time frame + the phone number used for the delivery by the SMS provider