An example of just such an easy-to-figure-out colloquialism is “不嘛,” which means something along the lines of “Nuh-uh!” or “No!” (as in the way a child starts off an argumentative sentence with “No it’s not!” or “No it isn’t!”)
A few things I should cover right out the gate: 红烧排骨和猪蹄 is the name of a dish. 红烧 is a way of cooking a dish where food is slowly simmered in an oily red sauce. On Chinese menus you’ll often see 红烧 followed by many different things. One of my favorites is 红烧茄子 – Eggplant Simmered in Red Sauce. So this dish, 红烧排骨和猪蹄, is spare ribs (排骨) and pig feet (猪蹄) simmered in red sauce (红烧).
One of the more difficult sentences here is 要是换成我吃那么多的话，现在写作文的早就是个饱死鬼了 – there are multiple points of difficulty here.
Let’s break this one down a bit. In the previous sentence, the author is talking about how much her brother can eat. Then she says, 要是换成我吃那么多的话 – the hard part of this is 换成, which means “to switch”, or in this case “to switch positions with someone”. In other words “If (要是) I switched positions (换成) with my brother (‘with my brother’ is implied, not said), and it was me that ate so much (我吃那么多)”…
The sentence continues: 现在写作文的早就是个饱死鬼了. I think the hard part of this is the way 的 is used. Beginning readers expect 的 to either follow a word to make it possessive (我的, 他的）, or to show that something’s being described (“红色的箱子”). In this case, the author actually left out the word being described, which is 人. This part could also be written 现在写作文的人: or in other words “the person writing this essay right now…” This is a very informal way of talking. We continue: 早就是个饱死鬼了 “Long ago (早) would have become (就是个…了) a full devil (饱死鬼 – someone who has eaten until they are overstuffed – google image result for 饱死鬼 gets us this).
So all together: “If I was the one who’d eaten that much, the person writing this essay [me] would have long ago become overstuffed.”
And one last thing: the older brother at one point calls the author 臭乐乐. I understand this translates to “Stinky Lele!”, so we can assume here that “Lele” is the author’s name. This is probably a girl’s name, so we assume the author is female.