iCloud A/c password forgotten

Dear,

Please help me!

I was using iPhone 4s before which was given to me by from sister & the iclound a/c in that phone was her husbands. I didnt really notice that & started using it not really understanding that all the data was been saved into his icloud a/c. (contacts, mesages, etc).

Now that i have baught a new iPhone 6, i wanted to transfer the data from 4s to i6, but dont know the password of that a/c nor my sister & her husbands remember. they dont even remember the security questions.

i am trying every where to see if i can do something without any luck.

Please can you suggest me, how can this be resolve, all i know is the icloud a/c id.

Pleas this id contains important data which i need to recover.

Thank you in advance!!