I assume you're talking about those thick padded fabric pants instead of pull ups? I never used those. I started with pull ups with my oldest son when he was close to 2. I tried underpants several times, but he just kept wetting them, and when you're cleaning up your 5th accident of the day, it's very hard to not get mad or frustrated with your kid. :) After a few attempts at that, I had to "bribe" him to go potty and keep his underpants dry. I wrapped up a boy and made an excel spread sheet and every time he went potty and kept his pants dry, we put a sticker on it. Once he filled in all the spaces, he got to open the presnet. We started with 5 stickers, and then 10 . . . He was a nightmare to potty train. He was 3 1/2, almost 4 before he was completely potty trained, and well past his 4th birthday before he was dry at night. So, basically with him, I went from diapers to pull ups to underwear when he was ready. The pull ups were easier than diapers in encouraging the potty so I didn't have to get anew one if it was still dry.
Because of the endless headaches, I never really brought it up with my second child. I guess I was in denial. He'd go when I'd take the oldest or before baths, but I never pushed it or anything. I was going to work on it this summer. Well, right after his 3rd birthday, he just decided to go to the bathroom. And he did and has had no accident whatsoever. He just went from soaking his pants all day to absolutely nothing, even at night. So he went from diapers straight to underpants.
I think with my first, I was feeling pressure with other people telling me to get him potty trained that it became me wanting him to do it rather than him wanting it, so it was a power struggle. I think that's why it wasn't really successful and took so long. With the second, I never pressured or anything, and he honestly did it completely on his own. Since your son is showing interest, I'd go ahead and try underwear, and if it's not successful, just put it up and try it again in a few weeks. I wouldn't pressure your daugther. Just make such a big deal when her brother goes, maybe she'll want to do it too.