I sincerely hope you take this advice to heart, pre-marital counselling is a must in my view and definitely brought up (luckily only a few) minor things my husband and I missed (thinking we had all areas covered already).

Something also that really stood out to me, other than the huge lack of compromise and excluding your sister/family in general it seems; that your fiancée and her Mother are "mad at you for stealing her spotlight". This is 100% your day too, having your niece be a part of it (since her entire family are on both sides of the wedding party already which sounds like you have been extremely flexible), should not be an issue in the least. Having 2 flower girls also isn't against tradition in any way...but still, if she's only wanting one, why does it have to be her niece and how on earth would either the both of them, or rather just your niece take away from your fiancée's spotlight?? I'm seriously trying to understand her logic here, for a day that is again, for both of you.

One last thing, it is very problematic that her Mother is already involved and mad at you, a spouse who runs to family to complain about what should be between a couple, is something that really, really needs to be discussed.