Unless a contrary intention appears from the will, the bequest to your mother would fail by reason of the fact that your mother did not survive your god mother and the house would be included in the residuary bequest to your god mother's cousin.
If your god mother wants the house to go to yourself and your sister in the event that she is predeceased by your mother, the will would have to contain a so-called "gift over" clause.
As a general comment, I think you should thread carefully here. Perhaps gently suggest that your god mother revisits the terms of her will with her solicitor to ensure that it accurately reflects her intentions.