Is it normal i'm in love with my best friend but i'm in a relationship ...

For the purpose of the post my name and the girls names are not correct

I've been friends with this girl (Becca) for 6 years, within the past year we have become very close,
I am currently in a relationship with a girl named Sarah for past 2 years.

Recently I have been having many arguments with sarah and consequently I have seen a lot of becca, I have always been attracted to becca but never said anything as she has always had a boyfriend or talked about how fun it was to be single ...

I live at home with parents still but they are on holiday so consequently I have have had the house to myself,
Whilst they have been away becca has been round as I have been having arguments with sarah,
The first night she came round we ended up kissing,
Since then we have been texting everyday and flirting within those texts, when she came round last night we ended up having sex ...
None of this was down to alcohol,

I want to start a relationship with becca, but I do also want to stay friends with sarah,
Just to complicate the matters more I don't know if I should tell becca how I feel as I don't want to jeopardise our friendship, she brags about being single so why would she want a relationship?

I know indefinitely I love Becca and its not just a crush, but I just dont know what to do
And how to break the news to sophie?
How would I tell becca I felt if I was to do so?

All advice would be majorly appreciated