What is the purpose of 两 ?

Hi all,

I am learning Chinese at university for an elective subject.

This week we were introduced to numbers and family members.

For example 我有两个哥哥

I'm sure there is a reason but when I asked my Chinese friends they had no idea why 两 is used instead of 二.

As far as I know every other number of brothers a person could have would just be (that number) + 个

So what's the issue with 我有二个哥哥?

Thanks in advance for any responses!

谢谢

好好学习，天天向上