You and Law are in a relationship, He was showing you how to conduct a blood test to find out if people are related in a way. He used his and Luffy blood he had from taking care of his wounds once. Not thinking anything about it. Cause there was no way he could be related to This Straw hat guy

Law: okay... And now... Nothing will happen! Cause...

Law stopped mid sentence as he was analyzing the DNA more closely

He stopped to breath for a second, finding out that Luffy, the guy who always was clingy to him and who annoyed him. Was his real, one and only blood related Little brother

After a while, you, Law and Luffy were on the Sunny. Law had to tell them, not only Luffy but also his crew

Zoro: what is it? Why are you interrupting my sleep!

Sanji: don't you now say your stuff is so important zoro!

Zoro: huh? Who is the one who is always just in the kitchen?

Nami: guys shut it! Let's see what is so important that law hat to tell us!

Law put the blood test result on the table, everyone except Chopper didn't know what to do with it

Chopper: wait... WAIT! don't tell me!

Ussop: why are you freaking out so much chopper?

Chopper: it... Shows... That he's blood related!

Zoro: just spit it out already! We want to know!

Law sighs, before looking at Luffy, who was happily eating his meat, everyone wasn't prepared for what Law had to tell them

Law: I was analyzing my and luffys blood! Being 100% sure that we aren't related...

Luffy: i mean it's true! I never saw you in my childhood! So we can't be related!

Law: no! We are! We are blood Related Brother's!

The whole crew was shocked, didn't know something like that could happen

Sanji: So you 2...are really brothers?!

Chopper: the test is definitely real!

Law: i suppose you got taken away to another island when you are born... I didn't even know you existed till we meet that day...