What legal reasons? They're not legally bound to only accept cis women, its just one of their own rules. They don't have to go through the courts, they can change their bylaws to include trans women. Or whatever verbiage they need.

Unless I'm missing something, they have no legal obligation to keep the same bylaws as an organization. Then there's no basis for a terf to sue.

ETA: whoop, missed this part of the article apparently;

Green said the organisation wanted trans women to remain “part of the WI family” and that from April it would launch new “sisterhood groups”, open to all, which would be “a place where we will recognise transgender women as women and explore what it is to be a woman in the 21st century."

So if that's possible, then it seems absurd that the main organization could be at risk of legal action