There is a very good chance that this will fall under the windfall clause and will have to be paid into your IVA -- please check your paperwork.
Don't think of it as losing the money -- OK you will pay back all of your debt instead of having half written off, but you should benefit from not getting stung by the interest you would have beem paying. This will ahve the effect of drastically reducing your IVA term, as you won't pay more than 100% of the debts, IP's fees and, possibly, statutory interest. So you will hit that figure pretty soon and be debt free.
So, your mother will have given you the chance of a fresh start, and you will be £200 per month better off.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014