There's a difference between consciously and unconsciously knowing.
No, she didn't consciously know this - i.e. she wasn't aware of it.
Leia meant that the connection she always felt towards Luke now made sense - the one that she originally misinterpreted as sexual love, but realised was (appropriately enough), sisterly love.
Even when Leia kissed Luke she had other motivation - to annoy Han in particular - this, combined with a sisterly love she didn't know was in fact a biological connection, was enough for a kiss.
scifi.stackexchange.com/users/108/Tony Meyer
No.
She was not aware of the blood relationship at that time, though she certainly felt the connection that most fiction insists is there between twins.
She may also have felt a strong desire for him, as they are twins (and thus very similar in appearance, though the performers playing them are not).
People in general are attracted to people who bear a close resemblance to themselves, and they were separated literally from birth, preventing the Westermarck effect from occurring.
It's entirely likely that her 'always knowing' represents the way that the Force tried to guide her away from Luke, possibly directed thusly by the spirit of Kenobi.
scifi.stackexchange.com/users/656/Jeff
Not consciously anyway.
She might have "felt" it somehow when she was kissing him.
At least that's the angle they usually spin on those kind of scenarios.
scifi.stackexchange.com/users/1239/XQYZ
I'm sure I read somewhere that George Lucas didn't know they were going to be brother/sister when he wrote that scene.
scifi.stackexchange.com/users/1612/Phil Lello
Both Luke and Leia are, to some point, force-attuned.
This is proven when Leia is able to hear Luke calling out her name when Luke is hanging off of the antenna after he loses his hand in cloud city:
LUKE: Hear me!
Leia!
LEIA:
Luke...
We've got to go back.
LANDO: What?
LEIA:
I know where Luke is.
In fiction, there is often a sort of psychic link between twins, but in the Star Wars universe, because Luke and Leia are not only twins but force-attuned twins, we can see that a psychic link between them most definitely exists.
However, only Luke has been trained in the use of the force, having to explain the link to Leia:
LUKE:
There's more.
It won't be easy for you to hear it, but you must.
If I don't make it back, you're the only hope for the Alliance.
LEIA:
Luke, don't talk that way.
You have a power
I--I don't understand and could never have.
LUKE:
You're wrong, Leia.
You have that power too.
In time you'll learn to use it as I have.
The Force is strong in my family.
My father has it...
I have it...
and...my sister has it.
LUKE:
Yes.
It's you Leia.
LEIA:
I know.
Somehow...
I've always known.
Since Leia hasn't been trained in the use of the force but she is still a force-attuned individual, she recognizes some sort of a link between her and Luke but she doesn't know exactly what it is (as she hasn't been trained in recognizing the force and being able to understand what the link that she's feeling means).
When Luke tells her that they're siblings, this confirms the link that Leia has been feeling all along, so she states that she has somehow, always known.
scifi.stackexchange.com/users/55675/Paul Omans
Actually Leia is daughter of Anakin which means she has midichlorians.
She has jedi powers which never trained.
That's why she can sense but can not understand if the sense is right because she is not trained.
And the kiss was to make jealous of Han.
scifi.stackexchange.com/users/452/pichoscosama
Well she didnt know she just felt the connection between them even though thought is fun to play with (third time I watched them
I realized it and got a good laugh out of it)
scifi.stackexchange.com/users/17456/Magn3715
The kiss was genuine romantic attraction tempered with a motherly/big sisterly approach.
I don't believe that Lucas decided that Leia and Luke were siblings until after The Empire Strikes Back.
In the episode IV Leia was torn between her attraction to the shy, sensitive Luke and the brash, macho Han.
She takes the initiative with Luke because she realizes that he is too shy to come onto her himself.
Lucas created a beautiful triangle complicated by the strong brotherly bond between Luke and Han.
scifi.stackexchange.com/users/49102/Skywalker_297
In ROTJ Leia says    "somehow I've always known"   If she has always known then did she know when she kissed him?
Did Leia know Luke was her brother when she kissed him?
scifi.stackexchange.com
<star-wars>
scifi.stackexchange.com/questions/3013
scifi.stackexchange.com/users/1600/Chris_O
Leia meant that the connection she always felt towards Luke now made sense - the one that she originally misinterpreted as sexual love, but realised was sisterly love.  She was not aware of the blood relationship at that time, though she certainly felt the connection that most fiction insists is there between twins.   George Lucas had not, apparently, decided Leia and Luke were brother and sister at the time the kiss scene was written. Hence, Leia did not know Luke was her brother when she kissed him.
Leia did not know that Luke was her brother when she kissed him, although she felt a connection between them. Apparently, the writer, George Lucas, had not decided they would be brother and sister at the time that scene was written.