I imagine the chords your brother found years ago are the same chords that guitarists use today.
There's several methods to go about "learning how to jam", the one I most support for people starting out, though the people you're playing with might not support it as much, is to just play. Just noodle around with your fingers, if it sounds alright, keep playing that, if it sounds bad, don't play it anymore. Pick some notes that you notice sound good and try putting them together in sequence. Then try to make an interesting, although fitting, rhythm to put these into. And you're well on the way to building a great ear and developing your own voice in a jam.
The other, more sound way of not annoying people, yet not as useful in the long run, is to learn chord spellings and their shapes on the bass, figure out what chords they're playing in the jam, noodle around with the notes in the chord, eventually you'll get a feel for it.
An even safer method is to know where the 3rd, 4th, 5th are in relation to any note on the fretboard (hint: they're always in the same relative position) and then figure out what the root is to whatever chords they're playing, play that first and branch off into the other three and then when you feel comfortable with that then just keep messing around further.
Oh, and remember that no matter what notes you play, try to keep the rhythm interesting, or at the least, safe. Ideally you should be listening to the drummer more than the guitarist or anyone else.
At least this is how it's sort of worked for me.