I suppose it very much depends on the colour and structure of your walls.
Often it is not the best thing to try to cover things up in an attempt to "make it disappear". Sometimes it is better to make what I call "an honest repair" where things "don't disappear", but the repair is tidy.
Maybe drill out the holes slightly to get a clean edge, and then insert a plug
along the line of this? These plugs come in all sorts of colours and sizes.
Alternatively you get a large photo of your mother in law
- put it in a frame and hang it so that it covers the holes
Sorry - I just couldn't help my self there.