Er, no. OP's sibling-in-law is and always will remain female, a type of woman, and as such, heavily dependent on women's rights.
Example: if they don't undergo a hysterectomy, they'll retain the ability to get pregnant, and will need to be covered by all the maternity rights granted to women. That was one of the points covered by the UK SC in its judgement, and which the TRA's would have happily ignored, thus leaving transmen with no maternity rights.
If they don't have a hysterectomy and don't take testosterone (or taking testosterone doesn't have enough of an effect), they'll keep having their periods, something that men don't have but so many women do.
They'll also have to consult a gynaecologist regularly, to take proper care of their female reproductive organs.
If they do undergo a hysterectomy, they'll have to take the same life-long care of their female body with no more uterus than other women who have a hysterectomy do. Their body is not, and never will be, a male body. It will always be a female one, not matter how modified.
Conclusion: OP's sibling-in-law may identify out of being a woman as much as they want, but they can never identify out of having a female body, with all the consequences that come with it, and the need for women's rights and accomodations.