Neither of those two phrases strike me as particularly idiomatic. Using ‘first’ and ‘second’ is not a common way to refer to the comparative ages of your own siblings. A parent might do that (e.g. my first child = my first-born = the oldest of my children), but generally speaking, a sibling would not use words such as ‘first’ and ‘second’ that way.
It would make more sense to first establish the fact that all of your brothers are younger than you are, for example, and then to separately compare age/birth order (“my oldest/eldest brother”, etc.) I happen to have three younger brothers, so I might tell you something like this:
- I have three younger brothers. The eldest is named Charlie. Ben is the middle brother, and Jeff is the youngest.
I also happen to have have a younger sister. The birth order of my siblings (and me) is this: Me, Charlie, Ben, Jeff, Jane. So, I might then also tell you this:
- I also have a sister. Jane is the youngest in the family.
In this case, even though Jane is the youngest of my siblings, I will still refer to Jeff as “my youngest brother”.
No, this only refers to the boy who was born last. It says nothing about sisters. (See my last example.)
Now, if my sister (Jane) were older than Jeff, then I might say this:
- Jane is the second youngest (in the family).
This is idiomatic, but it says nothing about whether you are older or younger than this particular sister.
This is not really a common way to talk about this.
This may well confuse people.
If you have more than three sisters, you might refer to them as “my eldest sister”, “my second-eldest sister”, my youngest sister", “my second-youngest sister”, etc. If you have exactly three sisters, you might say “middle sister” for the one between the oldest and youngest of your sisters. However, none of these include information about whether your sisters are older or younger than you are.
If you talk about “my big brother”, then you are referring to a brother who is older than you are. Without further context, it suggests that you have only one brother who is older than you are.
If you talk about “my little sister”, then you are referring to a younger sister. Without further context, it suggests that you have only one sister who is younger than you are.
[size=75]“Big Brother is watching you.” ~ George Orwell[/size]