I really wouldn't lend him the money.
Withdrawing cash on a credit card can negatively impact your credit score, people who borrow cash (paying an 8% handling fee) on their credit card are usually doing it as they are pretty desperate. This will remain on your credit history for 3 years usually - mortgage lenders would not look upon it favourably (even with the explanation - in fact the explanation may lead a lender to believe you are financially assisting family thus reducing future lending).
The reason a high street lender won't lend him money is because He is a bad risk, commercially they don't see it as being viable. That's why he is going to loan sharks.
What were his debt repayments meant to be? If he can afford £400 a month and only owes £4500 why is he in arrears?
Why is his home at risk? From a lender or a loan shark?
I speak to people in depth about their finances and I doubt he is telling you the entire story.
If you are determined to help him out (I wouldn't) I suggest you get him to disclose everything to you in detail and then Say you want him to obtain his credit file (experienced and equifax) (and sil credit file although thAt will prove impossible as he cannot obtain on her behalf) and you look at it on line with him (so he can't hide pages - yep I have had people do this) to ensure he is telling the entire truth. If he was honest there will be NO anomalies. I would also recommend you obtain 6 months bank statements for all accounts showing on the credit file - gambling is rife at present and often people use a secret account.
Also if you are determined to assist, pay off the creditors - don't give him the cash/cheque and get a loan agreement written up.
You may find he doesn't want you to tell DH as SIL has confided in your DH what is really going on... I really would tell your DH and not lend any money.