[Recliner]

First thing, if people are raising $15 preflop and others are calling, he should have raised $15. If they are going to call $10 they will call $15 and that gets more money in the pot when he is a favorite.

On the flop if he decides to play the correct play would be to push and I'm going to assume both players at least call his push, giving him 2.3:1 for his money on the play. So he has to win 45% of the time for it to be profitable.

Now for the important part, which you totally left out. What are his reads on the two players? Are they drunk? Are they playing nearly every hand? How often do they raise? How is their post flop play?

Because it’s such a low limit, I'll go with them being total donks that like to play every hand. As such there are 5 different scenarios that would make the most sense: (Quick note, I don't know how to do one random card in pokerstove, or if I can even do one random card so I just picked some cards to go with the threes and nines)

they both have random hands............. 70% to win
3cTh and a random hand.................. 9%
both have a 3, one also has an over card 9%
9hKs and a random hand.................. 70%
One has an A the other a K.............. 75%

So the majority of the time he is either way ahead or way behind, so now you have to assign the probability of each of these events happening. (This is where having good reads comes in handy.)

There isn't a big enough difference in the chance to win if one of them has a 3 or if both do so we will think about them as the same event with a 9% chance of happening. Player 3 doesn't have a 3 because he'd try to slow play it. Player 5 might, and be trying to slowplay. As there are already 2 of the 3s out lets say one/both of them has a 3 25% of the time.

The other three possibilities we'll treat as your brother being a 71% favorite and will happen 75% of the time.

Now multiply the weights of the events happening:

.25 * .09 = 0.0225
.75 * .71 = 0.5325
for a total of 55.5% to win, which is greater than 45% making it +EV to push.

In fact it is +EV to push up until the chance of them having a 3 reaches 43%. This also doesn't take into account both of them folding or one of them folding for $30 more into a $173 and $203 pot respectively. I'm sure I left out some stuff that may have been of minor importance but I think my analysis was good enough to say that he should have pushed his chips in.