A moral dilemma about special needs

Here's a situation for you to think about. I will probably post again about related matters, but let's stick to this issue today.

It's not hypothetical.

There is a young woman of 20. She is profoundly deaf with little knowledge of sign language. Her mental capacity is largely unknown (this will be the subject of the other post I threatened above). She has a boyfriend of roughly the same age. He has learning difficulties. Neither of them will ever be able to live independent lives as a consequence of their own limitations combined with the provisions of the country in which they live.

They want to have a baby. Her older sister recently became a single mother, and that got this young lady's broody hormones going. Her family has tried to point out that she is not in a position to raise a child, but she and her boyfriend don't have the ability to follow this line of reasoning. In the UK, they would probably be given all sorts of welfare support, but this is not a provision available to them. She argues that her sister is single and unemployed, so what's the difference?

Up to now, her mother has taken care of the contraception issue. Now that her daughter is starting to become adamant about having a baby, the dilemma arises: feeling fairly certain that her daughter does not have the ability to raise a child, and that the boyfriend is even less capable, does the mother infringe on her daughter's human rights and have her sterilised? What about the human rights of the hypothetical child, for whom no provision will be made by the state?

The reason I mentioned the woman's age at the beginning of this post is that, within very few months, she will be 21 and legally an adult, at which point the whole issue of her mother's power to impose her will becomes a thorny matter.

I was discussing this issue with a colleague whose girlfriend, a human rights lawyer, is writing a paper about the compulsory sterilisation of disabled people in Kenya. The case study for the paper involves a mentally competent, blind woman who was given a hysterectomy with neither her permission nor her knowledge after the birth of her fourth child. She only discovered that the surgery had taken place at all when she developed complications.

Where is the line between these two cases? Who makes the choices, and who gives that person the right to make those choices?