Rashi d"h lomar shemimaet (nidda 44b)

I just want to summarize this Rashi so that I can be sure that I have it clear in my own mind. I'm going to use the example that the father left 48 zehuvim instead of 6 just so that we can avoid fractions. We'll call the brothers Reuven, Shimon and Levi.

• Just two brothers with no baby in the picture: Reuven gets 32 (double) and Shimon gets 16.
  
• Levi is born before the father dies but then dies immediately afterwards: Reuven gets 30 and Shimon gets 18. That is because first you divide it between all the brothers with Reuven getting double. So Reuven starts with 24 and Shimon and Levi each have 12. There is no bechor for Levi's portion so they each get 6 of it.
• Levi was born after the father dies and then dies: Reuven gets 32 and Shimon gets 16. This is because you ignore the fetus when it comes to determining the bechor's portion. So the bechor gets 1/3 for his "double" portion. Then you divide the rest evenly between the three brothers so each brother gets 10 2/3 (ok, I couldn't completely avoid fractions). Now Levi dies so his portion gets divided evenly between Reuven and Shimon and they each get the same amount as they would have had Levi not been in the picture at all.