The unborn baby has rights too.
No, the unborn baby does not have rights. It's life, and it's human life, but it does not acquire personhood and the rights that go along with that until it is born. And that is how it must be if women's human rights are to be upheld.
If you want to see the alternative, look at parts of the US where women are prosecuted and imprisoned not only for ending, or attempting to end, their own pregnancies, but for miscarriage if there is anything about their own behaviour which might have contributed.
The vast majority of terminations take place within 12 weeks. If we want to prevent late terminations we need to make the whole process easier and quicker, not further restrict women's access to late abortion.
If your SIL would honestly have a late abortion just for fun I cannot see how forcing her to continue with an unwanted pregnancy could possibly lead to a good outcome for anyone 
There was a research doc I read once (I think it was linked earlier in the thread?) of reasons (aside from foetal abnormality) that women sought late abortion. The women's situations were all awful and harrowing.
I understand a debate is unlikely. I understand any debate would be unlikely to result in a change of law. I understand ths is not a formal coalition.
I am still horrified this is even being suggested and I am disgusted that any mainstream party in this country is giving the DUP with their vile backward views any sort of nod of respectability.