Self raising flour is plain flour with a standard amount of raising agents added. You can substitute SR for plain plus baking powder, you just need to get the proportions right. I just checked my baking powder container and it has a ratio of 2 tsp to 1 cup of flour. Where the recipe calls for plain flour plus baking powder it would most likely be due to the need for a different level of raising than what SR flour gives, so there might be 1 tsp added rather than the 2 needed to make SR flour. The would be no reason why substituting would have made your mum feel ill. The texture might be different, I suppose that could have done it. I have substituted many times without a problem and I am sure when I studied professional cookery that they taught us something about it. Sometimes I find it easier when I am not baking a lot to just have plain flour and baking powder on hand, that way I can bake anything.

As for sugars, best not to change them for the reasons rayray mentioned. I tend to hunt around for a recipe that has the ingredients I have one hand, rather than substituting something like sugar.