KOUMA RYODACHI wrote:

I think we discussed this already, but how can they be brothers if

they don't

share the same surname?

This one is very simple to answer.

Remember that when a marriage takes place in Japanese society, and
the daughter wed is the heir to the family estate and holdings, then
in order for the husband to gain ownership of the land and holdings,
he must assume her surname.

Therefore, Genma could very well have been born Masaki Genma, and
changed his name to Saotome Genma after marrying Nodoka.

It's not entirely impossible.

Particularly since Nodoka's katana indicates that her social status
is very high, while Genma isn't exactly the classiest man in Japan.  
So does that mean that Ranma will become Ranma Tendo?  (Although 
strictly speaking, Akane is only the heir because she is going to 
marry Ranma.)