I am a den leader of my son's den (Tigers last year, now Wolves), and have been bringing my younger son (5 now) to meetings with me every time. I make sure that he brings something to play with, and he is occasionally included with the scouts, such as at snack time and at our closing circle, but does not do the crafts or games or enter into our discussions. I explained to him that this is his brother's meeting, and that he will have his chance when he is old enough. This has always worked fine, and not caused problems for me, him, or the scouts. Occasionally other siblings have come with their parents, and we have never had a problem there either.
If I were in your position, I would welcome the participation of the mom and not worry about the presence of the girl unless trouble develops. This is not favoritism, since you have not excluded others. As far as everyone bringing their kids, don't worry about it unless it happens! If there are truly too many siblings attending for the space available, you can always have a talk with the parents at that time and explain that you need to make a change, and why.
