Hi Tinkabell, as Squirrel said, most of us here on MN have all been through this in some shape or form and it really does help to talk on here about it, honestly. Not a day goes by when I dont wish that I received a bit of love, affection or even just respect from my stepkids. I love them unconditionally, but then I dont have kids of my own to share that love with as you do and I increasingly feel that I have my nose pushed out of joint and my feelings and requirements disregarded.
My DP has always paid his maintenance and paid for other things and sees them at least once a month, despite living over 200 miles away from them. I know this is nothing like the amount of time you have them so I guess its a bit different for me. As Squirrel said, BMs can be a total nightmare and a lot of us on here have had horrendous situations with BM from verbal harrassment to physical threats and abuse. We put up with an alcoholic BM for too long before Solicitors got involved. Thankfully we now have no contact with her other than in writing and it suits us perfectly.
Unfortunately as horrible a situation as you are in, it is important to ensure that the children do not suffer (yours and BMs) in any way. They are the innocent parties in all this, im sure they would not choose the situation they are in either. However, perhaps you could sort something out with DP to get her to wind her neck in a bit about her insistance on them having everything they have at hers? No more should be spent on his kids as are spent on yours. Why are they having 2 of everything? I can understand it at Christmas, my stepkids love the fact that they have 2 Christmas's, but we do not compete with the size and cost of presents as although we both work 50+ hours per week, she does not work (legally anyway, only on the side cash in hand) and gets all the benefits under the sun she spends a fortune on the kids and we can only afford so much with CSA payments aswell. DP however, does try to make up for it in other ways and sends the kids little treats each week, like sweets, comics etc. or even a letter to let him know he is thinking of them.
I probably havent been much help, but all I can say is that it IS hard, damn hard, even for me without kids of my own, and although as you quite rightly say "they are her children not yours" they are also your DP's kids and you have to try and forget that they are a part of her aswell.
Unfortunately kids cannot choose their parents as we all know, and they certainly cannot choose their step parents! You can guarantee the situation is probably equally as hard for them and they have no say in any of their situation. Children pick up on atmosphere and tension and this can make them worse and unhappy. You sound like a lovely kind stepmum who has got down with the whole step parenting situation, this is understandable and it happens to us all. Take a long deep breath, paint on a smile and remember why you are their stepmum and how much you love your DP.
[[[[[[hugs]]]]]] xx