Can you take it as an AS? That would be the best solution.
As a Lit student, I want to encourage you to take it (I've personally loved it so far!) but A Levels are HARD and you've picked 3 tough subjects already. You've already got your work cut out, I imagine it would be really difficult to take a heavy subject like Lit on top of it. If you're being set an essay a week for English (usually about 1,500 words long) including research and writing time, you will struggle to keep on top of your science work. You also need to think of the fact that you'll have an awful lot of reading to do which might take up your free periods and some of your holidays. I'd worry, for you, that you might be sacrificing A* grades in your other subjects if you take it.
That being said, if you feel as if you have the motivation and capability to keep up with the workload - having 4 subjects, especially one that is a bit of a contrast to your sciences, sets you apart from other candidates. It might also be nice to be doing something a bit different as well!
If you end up not taking it, you can always keep being an active reader and do creative writing in your spare time. There are other ways to keep your interest in it going!
I'd also like to say that A-Level English Lit is pretty different to GCSE - there is far more emphasis on historical context and literary criticism and theory. So, even if you enjoyed it at GCSE, it might not be your favourite at A-Level. Not to mention, the amount of texts you have to read more than quadruples.
What ever you choose, I hope you do well. Good luck sis X