Why do my brother and dad forget about me?

- With your brother, it largely depends on your & his specific age and personality - if there's a large gap, you're not quite on equal grounds; you may have a relationship based on some shared interest, or if he wants to be your mentor (which varies with time), and later on when he sees one can never quite rely on anyone outside family.
Whether he'd mention you to his gf says more about their relationship, most likely.
- With your birthday, it might be kind of a guy thing, not remembering birthdays, anniversaries etc (that's part of what a wife is for!).
- Same applies to the speech, if it hadn't been written down ahead and addressed many people in his life.
So, overall with your father, you should take other aspects into consideration - how he treats you directly.

Lastly, remember that many people don't have a family to speak of nor to... try not to nitpick their faults, appreciate what you do get, and give back when you can. If you wish to be remembered, strive to achieve something memorable. It takes much time and effort, ought to keep the mind occupied.