I wandered lonely as a cloud… Except, he didn't. William Wordsworth had his sister Dorothy along with him. And she may have had more influence over her brother's writing than is generally thought.

When we were in the woods beyond Gowbarrow Park we saw a few daffodils close to the water-side. But as we went along there were more and yet more; and at last, under the boughs of the trees, we saw that there was a long belt of them along the shore, about the breadth of a country turnpike road. I never saw daffodils so beautiful. They grew among the mossy stones about and about them; some rested their heads upon these stones as on a pillow for weariness; and the rest tossed and reeled and danced, and seemed as if they verily laughed with the wind, that blew upon them over the lake; they looked so gay, ever glancing, ever changing.

So who wrote the daffodil poem anyway? Were they actually Dorothy's daffs, lightly versified by brother William?

Dorothy left her journal on the kitchen table at Dove Cottage, for William and their friend and fellow poet Coleridge to pinch bits out of as they wished. She seems to have been quite happy with this role as raw material. In William's Guide to the Lake Country, two final chapters, on Kirkstone Pass and Scafell Pike, appeared as if written by William himself – in later editions, credited to 'a friend'. In fact by Dorothy did them.

Even as brother and sister, Dorothy and William were unusually close. Separated in early childhood, they were reunited in their early 20s. It's not uncommon for such separated siblings to form a physical, incestuous relationship. There's no evidence for that between Dorothy and William. But the night before William married Mary Hutchinson, Dorothy wore the wedding ring on her own finger: and she went along on the honeymoon…

Given they lived 200 years too early for OK! magazine, we'll never find out the truth about just how close they were, or just how much of Willie was written by Dot. But quite apart from her collaboration with her brother, Dorothy has also left us her solo contribution to Lakeland writings – the diary she kept at Grasmere from 1800 to 1803.

During this time she explored the waterfalls of Easedale; went up Deepdale Hause to catch the daffodils over at Ullswater; and must have crossed Loughrigg a hundred times – it was her route to Clappersgate to collect the mail.

Dorothy in her day was a considerable fellwalker. In the last days of 1799, as they moved into Dove Cottage, she and William walked, in the snow, from Richmond at the foot of Swaledale to Keswick, in two and a half days. She was 48 when she made the first recorded female ascent of Scafell Pike – and by a leg-stretching route too, starting and finishing half way down Borrowdale, at Rosthwaite.

But it's the little things that she sees, and records in her diary. The progress of her green peas. Coleridge's tummy upsets. But also, Moonlight on Rydal Water. The sound of the cows carried across the water. The sparkling waterdrops above Greenhead Gill. Plus, of course, those damn daffodils.

• Read more about Dorothy Wordsworth and other unjustly overlooked women walker-writers here: