Sorry, this is a very difficult and upsetting situation but imo, you are in a no-win situation,
I have also lent money to a family member - they always 'needed' wanted the 'loan' in full, in one go, immediately. This happened more than once. I would get most of it repaid, piecemeal, bits and bobs all over the place which was no good to me as it just goes in your purse and not back in the bank!! Finally, I had to say no to a request to guarantee a loan
. It was damn hard to say and to enforce but it finally made that person face up to their own limitations with regards to money.
I think you need to look at your sil's situation, in real terms, with regard to getting your money back. She is on benefits, she has failed to pay anything other than £50 back (so owes you £1050), it is unclear from your post if her DH shows any sign of contributing to the repayments (if he is in a job???). Really, what is the chance you are going to see any of what it owed to you??
I do not mean to come across as harsh - I truly understand how you feel. I just know from bitter experience that you are unlikely to see much, if any, of your money and it is likely that feelings will start to run high if/when you address this issue with her. People are notoriously bad at realistically working out what they can afford to save/spend/pay back.
To that end, you need to decide whether to pursue this - even if you just want to get your point across
or if you want to chase the money? If it were me ... I think I would, at the very least, have to let her know exactly what damage / problems she has caused through her actions.
Sorry for the long post. Good luck.