She and her DH have been incredibly thorough and on the ball with contraception from Day 1. They don't want anymore DC as of yet.
Why does she keep on getting pregnant despite the use of contraceptions?
The first time - Due to the mirena coil. She fell pregnant 2 months after having it put in. She decided to have the baby.
The second time - The implant in the arm. 3 months of having it in and bam, pregnant. Termination at 4 weekish.
The third time - Condoms AND the pill. Pregnant again - termination again at 4ish weeks.
4th time (now) - Withdrawal, condoms and different type of coil. Her and DH have decided to keep pregnancy and have DC2.
I went to the GP with her this morning and she sobbed, asking why this keeps happening as she always takes precautions and isn't ever flimsy about it.
She isn't lying, she isn't being 'daft'. My sister sincerely worked hard to prevent pregnancies. The GP has quite basically said 'well abstinence is the only 100% success rate".
But why does this keep on happening to her? 
I was going through a lot of child bearing related sadness through a lot of these pregnancies, and even I sympathise and know something needs to be done.
She laughed over tea even and said it's funny because her and her DH aren't even that active anymore due to him usually working away!
Can anyone suggest anything at all? Why could this be?