I have no twins so I'm kind of at a loss for how to help DH with this situation. I don't even know if the girls being twins makes a difference.

DH has 3 kids with BM: SS19 (great kid) and then the girls, who just turned 13. I'll refer to them as SD1 and SD2. All 3 were conceived with donor sperm because DH is sterile. DH was a stay at home dad with the kids from the time SS was born until about 8 years ago when DH and BM divorced. BM had a job that pays really well so she was happy to support the family while he took care of the kids and the house. However, BM is also an addict, first alcohol, then opiates and finally adderall. She was in rehab three times and when they split, DH had full custody until BM had clean drug tests for 2 years. They've had shared custody for about 6 years.

SD1 is apparently like BM personality-wise. SD2 is more like DH. I think DH had always been a little closer to SD2 than SD1.

Over the past year, SD1 had been refusing to spend as much time with DH. Lately it seems as though she will text DH and tell him she's not coming over for various reasons, such as her mom is going camping and she wants to go along or she needs to stay to help BM with some chore around the house. I've encouraged DH to tell SD1 that he will not discuss changes to parenting time with her, only with BM. When he expressed to BM that he would like her to support his parenting time, BM said she'd never tell the kids they weren't welcome at her house.

This is tough because what do you do? Force the kid to come over and have her be resentful? Let her stay with her mom and probably eventually have little relationship with her?

I suspect that BM is engaging in alienating behavior but am at a loss for how DH can combat that. BM has much more income than DH so she's the "fun" parent who buys the girls stuff and can take them to do things. DH tries to engage with SD1 but she's often sour and rude to him.

The other part of this issue is SD2. She's like the scapegoat at BM's house and it's obvious SD1 gets preferential treatment. For example, at BM's house, SD1 has a queen size bed and a tv in her room. SD2 has a twin bed and no tv. I realize there could be circumstances that led to this but I'd think with twins especially, you'd try to be equitable. 

So, SD2 prefers to be at DH's house but I think she also plays up her perceived mistreatment at BM's house. I'm not saying she's not mistreated compared to SD1 but SD2 also has a tendency to exaggerate. I think at times DH goes overboard babying SD2 but he's gotten much better since I pointed out some of her behaviors the other month. The three of us went on a day trip and she repeatedly interrupted and talked over me, and has been bossy and argumentative. DH had heard about this behavior before and apparently SD2 has few friends because of it so he's been addressing the behaviors. 

Anyway, I'm hoping the hive mind here may have some ideas for dealing with one twin that seems to be pulling away due to possible alienation and keeping the other twin from using that to elevate her own status at DH's house.

Also, the girls do NOT get along. SD1 is nasty to SD2 and SD2 is nasty right back. I get that siblings don't always get along but I can easily imagine these two never having a relationship if it were allowed. I thought twins would have a special bond but not so with these two.

Thanks in advance.