I'm going to tell you what you already know. BM (Birth Mother) is jealous. Jealous of another woman mothering her child and possibly even jealous of another woman marrying her former husband.

And she is throwing a plain old fashioned temper tantrum.

My skids BM STILL tells the girls that she can't afford to buy them things. So you make the decision to either pay for it yourself or not pay. In the past I've paid for entire birthday parties or a season of teeball when BM was being moody about it. Sometimes she pays, sometimes not. In all honesty when she choses not to pay for an extra curricular activity, DH simply reminds her of this when she is asking for anything 'extra'...."Well I paid for teeball so why don't you buy them new tennis shoes...."

It all seems to even itself out. Before we had 50/50 visitation, we didn't buy school supplies either...and we stopped the whole clothes argument long ago. I just simply bought them enough clothes and toys to stay at our house, we just lost interest in arguing about it. She was never going to rationally discuss it and her only interest was to be a complete pain in my rump. If she feels the need to hold the clothes and toys hostage...let her.

My skids BM has over time simmered down quite a bit. Especially since we quit enabling her to constantly interrupt our household. DH rarely takes the bait when she's engaging in an argument and his favorite saying has become "I'm sorry you feel that way but I'm not going to argue this any longer. We are just going to have to agree to disagree..." My counselor taught me that one and it works like a charm.

Anyways, you sound like you have a lovely head on your shoulders and it's nice to meet you too!!!
"For every ailment under the sun....There is a remedy, or there is none;
If there be one, try to find it; If there be none, never mind it." ~ W.W. Bartley

I have seen it before and it happened to me. BM seemed just fine before we got engaged, then all sh(*(*&t hit the fan. So I think that is normal for yours to be acting out like she is. But specifically to your questions...

I have learned to make everything clear and document. SO. have your fiance, work out his agreement with BM. For example, what is at her house stays at her house (if that is what she wants) and everything at your house stays at yours. In many decrees though, what belongs to the children is theirs and they can take them back and forth as they please. We struggled with this for a long time. Everything would migrate to BM's Only crappy clothes would arrive at our house and I would shop endlessly to keep the kids in nice stuff because it would return to BM's and never come back here. I learned that in fact she bought them lots of new clothes too, but didn't want any of them at our house. Go figure, send your kids off for a weekend in horrible, ill-fitting, worn out clothes, just great for their self esteem. More importantly though is the message they are sending to their kids. Don't want to go too deep down that road on this post though. Any way, once the "rules" are defined, stick to it.

Also, let your husband defend himself to his daughter. He should tell her that ex and he have different income levels but that her well-being is most important. Let her know that the courts have worked out an equitable plan where he pays child support and that any concerns that SD has should be left to her parents. She doesn't need to worry, these are issues too big for little girls and that sometimes adults say and do things that are inappropriate. He could also explain why he divorced her mom and that even though they are not together, he loves her. I'm sure this has already been said a million different ways, but I learned that you can never say it too much.

Okay so, my final thought is, you have to decide whether or not you can afford soccor or whatever it is. When SD wants something, if you think it should be split, talk about that ONLY with BM not the little girl. If BM refuses, or makes a big stink, you two decide if you can afford it. This is really what is in the best interest of the child. If you can afford it, I would try to think of it as if this were your child and do for her as you would your own. It doesn't matter what free ride BM is getting or whatever. If you go down that road of ticking off what she is spending or not, you will drive yourselves crazy and probably have a lot of fights. If BM bows out, and SD really wants something, you are within your rights to find a less expensive program or make stipulations about her commitment etc. Whatever you do, don't let the child be involved in the debate, discussion or anything involving money or pitting one parent over the other. The simple answer is, yes you can play soccor, or if neither parent can afford it, well "I'm sorry honey your mom and I can't afford it". If that is the case, this is a great opportunity to teach your child to find ways to earn money for herself, or understand the way to manage a household.

Also, start a file, because one day, if you find that BM is making no contribution and you have continuously tried to share financial responsibility and she won't, the next time you go for a modification of child support, you might be able to lower it, or get more decision making rights. Who knows.

Anyway. This is the way it will be for you. Step-parenting comes with a lot of good and a lot of bad. Sounds like the good for you out weighs the bad so you are in good shape. How you two manage things with BM now will determine many things about your relationship with SD and BM in the future. My best advice, tread lightly, be kind, put the children's best interest first, and let your fiance do the talking. Stay behind and smile.