it should be somewhere here : http://svn.rathena.org/svn/rathena/trunk/doc/item_bonus.txt
* Race (r)
RC_Formless, RC_Undead, RC_Brute, RC_Plant, RC_Insect, RC_Fish,
RC_Demon, RC_DemiHuman, RC_Angel, RC_Dragon, RC_Player, RC_All
bonus2 bSubRace,r,x; +x% damage reduction against race r
then using the above item bonus,
bonus2 bSubRace,RC_DemiHuman,100;
or
bonus2 bSubRace,RC_DemiHuman,200;
you can try it I guess
What I mean is how to change their max resists?
I am not sure if I understand you right, however if you say resistance for race I guess there is no such thing as max resistance or min resistance of demi human since there is no inter relationship from one race to the other. There is no settings that a demi human can inflict more damage to brute or to any race.
I have tried to do a search and this concern might be related to this post at some point:
Damage Reduction for Demi Human
Other that this, when you say resist the one thing I remember about it will be elemental table, depending on your settings you can find them here.
For Renewal - http://svn.rathena.org/svn/rathena/trunk/db/re/attr_fix.txt
For Pre RE - http://svn.rathena.org/svn/rathena/trunk/db/pre-re/attr_fix.txt
On those table you can adjust the effects of elements from one another however be guided that not all Demi Human are neutral. Below is a list of demi human race monsters with different elemental properties.
http://ratemyserver.net/index.php?page=mob_db&race=7&mob_search=Search
If still this does not answer your question, please let me know.