Answered by SunniPath Answer Service Team
Answer:
Wa Alaykum Assalam wa Rahmatullah wa Barakatuhu,
In the Name of Allah, Most Merciful & Compassionate
One should join the Jamaat, even if it is in the final sitting. As for what to do as a latecomer, please see the following links:
Latecomer questions, giving salams by mistake
What Does A Latecomer Do? Also: About Durr al-Mukhtar
When does the latecomer recite the opening supplication?
What does the latecomer say in the imam’s final sitting?
With regards to praying with one’s sister, please see the following link:
Praying in Congregation with Family Members
Wassalam,
Sunni Path QA Team