why this sp dont work

create proc parentesco2
@a char(1),
@b char (1)

as
begin
declare @pa char(1)
declare @pb char(1)
declare @res varchar (50)
declare @res2 varchar(50)
select @pa=sujeto from persona where sujeto =@a
select @pb=sujeto from persona where sujeto =@b
set @res=
case
when @pa = @b then 'es hijo de'
when @pb = @a then 'es padre de'
when @pa = @pb then 'es hermano de'
else 'otro'
end
If @res is Null
If @pa is Null and @pb is Null
Set @res = 'otro'
Else
Begin
If @pa is Not Null
Set @Res = (@pa, @b)
If @pb is Not Null
Set @Res2 = @a, @pb
If @Res <> 'otro'
Set @res = Case @res When 'hermano' Then 'sobrino' Else 'nieto'
End
Else if @Res2 <> 'otro'
Set @res = Case @res2 When 'hermano' Then 'tio' Else 'abuelo'
end

In which rdbms? Got any error?

What are you trying to achieve? You are selecting from persona table but there is no loop/cursor processesing so the whole thing will run once for the last record on the table.

1. you need the following statement at the end of the proc

select isnull(@res,0) as result

b. Always have "Begin" and "end" for IF statements