Beth - I think some people may have got carried away on this thread and made some massive assumptions. The assumption you are the "Other Woman", the assumption you are grooming, are based on reading between the lines taking 2 + 2 and making 5.
I am uneasy about the assertions that step-parents automatically become status-less, faceless blobs with no sentiments, no ability to build a relationship with step-children that come into their life without being absolutely ham-strung, having no say in anything and whose opinions and input are worthless, not important, irrelevant. These are gross generalisations which may or may not have any relevance to your situation. I don't think anyone who believes that will ever be convinced otherwise, so I have not pursued it here, I respect people's views and opinions, however I definitely find that a very depressing and myopic view and doesn't take account of contribution that can be made, intelligently and commensurately.
Interestingly, nowadays I believe the law does provide for greater recognition for resident step-parents when they have lived with the DC's father/mother for a period of time. Marriage to the DSS's parent increases and reinforces that as being a permanent enduring (and hopefully loving) situation. I am not suggesting that there is any need to "use" that status necessarily, however it demonstrates that social thinking has move forward massively, step-parents aren't totally unimportant. Of course the DC's parents have their rights, but totally discounting the step-parent's contribution, so they become "pretty wallpaper" is abhorent.
There is in loco parentis, hence if your fiance has given permission for you to care for his DS, within reason, that should represent a trust situation and it may do more harm than good (especially if your DSS's mum is extremely negative) to continually text or phone every time - let's face it, there is nothing to stop her from thinking you are doing it to wind her up, when actually the reverse would be true. It is called "you can't do right for doing wrong".
The role of the step-parent can be rewarding both for the step-parent and the DSC's, it can also be a dignified, long-lasting and loving one. I know this, because my relationship with my DSS is thus, and I don't feel any sense of guilt to have made such a difficult relationship successful and to have done that without my DSS ever being dragged into the cross-fire (yes it has happened, but not at my instigation, and I have had to side-step so many times, I have lost count!). It is akin to walking on a tight-rope, not shadow of a doubt, it requires investment of effort, patience and the ability to detach sometimes and not take things too personally! Skin of a rhino springs to mind!
Looking back at my 7 years as a DSM, I would say the best approach is to play the long game, there is no need to prove anything, just be you, enjoy your life and build happy positive memories by becoming increasingly 'politically astute' about the situation and how to avert danger at all costs!