Get counseling for yourself if he won't go.

It's not the child's fault she's acting this way. It's how she's being raised. She's got issues that her parents are too lazy to deal with.

Document & take photos of her if you think she's being neglected. Maybe CPS needs to be involved.

First of all practicalities: you should not be cleaning up after her incontinence accidents, particularly not if you don't care for the little girl and it makes you more resentful. Dad should be doing it. I know it's unpleasant and frustrating, but honestly some children just genuinely have problems in that area and take a long time to be fully continent. Other children have problems in that area to express distress, stress or anxiety. Its up to the parents to address it.

My SD had a problem with incontinence till she was about 11, she saw a doctor and a specialist at age 7, no physical cause was found, and DH and BM were advised to downplay it and it would eventually sort itself out. There is a big diffference between downplaying it, and ignoring it and pretending it isn't happening. The later is not an acceptable approach, I know because DH tried it, as a separated Dad he felt awkward and uncomfortable talking to her about things that might embaress her and he did not step up to take care of the laundry in a timely way until I had a real go at him and told him he wasn't being fair on her or on me. Pull-ups at night are a perfectly acceptable solution to night time incontinence if she can still fit into them and if she's not made to feel bad about wearing them. Failing that Dad needs to buy a set of good mattress protectors and check her bedding every morning. He needs to talk to her about the importance of keeping clean, and changing and asking for help if she needs it. In all likelihood Dad feels a bit embarrassed about his daughter having this problem and your annoyance about it is making him feel like a failure because he doesn't know how to fix it, so he's lashing out. Perhaps if you can agree some practical measures to deal with it it would be less of an issue between you.

If SD talks about her mother or life before you, try not to take it personally. All small children talk about their mum a lot, and all children struggle when their parents divorce and remarry, she's is not your mortal enemy, she's a small child that understandably wishes her parents hadn't got divorced. You could be the fairy godmother with a lifetime Disney pass and she would still wish her Dad was married to her Mum, so don't see it as any reflection on you or how much she likes you. If it bugs you then make a perky response and rapidly change the subject.

Leave her Dad to do the parenting, spend some time doing your own stuff when she's there, but also try to find one nice thing you can do with and for her when she is staying. Play a game, do baking, watch a film, make something, anything which demonstrates some effort and care into your relationship with her. If you create some positive experiences it will make a big difference to DH, who may then be more receptive to your other constructive comments about things, and will help SD see you as a positive in her life.

Your frustration over the furniture buying is completely understandable. In his eyes he's just looking after his daughter but in effect he's helping fund the other home. You have to find some way to calmly explain to him how unreasonable it feels to you, but that may be challenging given you are in conflict mode at the moment. If you try some of the stuff above he may be more reasonable.

Here's the thing. It's pretty clear that the 7-year-old isn't ruining your marriage.

Your husband is, for allowing the behavior to continue.