Family related terms

Hi guys,

I am having a little trouble understanding the meaning, and the usage of these words. My professor lectures entirely in Japanese, so I couldn't understand her explanation. Some words do show up in the dictionary, but some don't.

一人っ子: This means the only child, right?
二つ上: More than 2? A family with more than 2 members?
四つ下: Under four? If I replace Ni and Yon with San, would it still make sense?
五つ違い: Difference of 5? No idea what this means...
末っ子: Don't know what this is, either.
義理の弟: Man of the house?

She also mentioned some other words like niban or sanban. If I ask her, she would just answer in Japanese. And using English isn't really recommended in this class, so I don't want to go there. If you guys can help explain these to me, that would be awesome!

Thanks in advance! :)

一人っ子: This means the only child, right? - yes.

二つ上: More than 2? A family with more than 2 members?
- no. Older by two years, is what it means.

四つ下: Under four? If I replace Ni and Yon with San, would it still make sense? -in the same way, it means younger by your years.

五つ違い: Difference of 5? No idea what this means... - once again, it is referring to age difference of five.

末っ子: Don't know what this is, either.- the youngest child.

義理の弟: Man of the house?- younger brother-in-law.

Whoops, "younger by FOUR years" is what I meant to write.