Better said might be Jesus knew what John's mission would be...and was wise to put his mother in his care along with the other disciples who met together, rather then that of family members who were so slow to believe.
1. We’re not talking about John...the topic is mary (sic)...and she was entrusted to his care....
You made the sub-topic family (regarding why Jesus asked John to care for her).
2. it had nothing to do with her mission.
You have not substantiated this is ANY way.
3. She completed that.
You have not substantiated this either.
4. Further she had other children besides Jesus as as we know.....
This has been totally proven false by all serious Bible scholars.
5. and likely grand children at some time.
Pure speculation based on false understanding of Scripture.