@Happyfarm I don't see how you can be in a relationship with him either. Leaving aside the issue of DC, he's fine with his toxic mother othering you, ignoring you, and stirring up shit between you and him. This is all her. She texted you those pics and sent the plant precisely to get a rise out of you. And you obliged her!
Then she gets to label you "crazy" and show 'D'H those texts you sent to prove it. To stir up trouble between you. Another tick for her, he agreed with her and picked her side yet again. Because make no mistake she's making him choose and he's choosing her every time.
That's why you've got no marriage IMO. All he cares about is keeping his mum happy, she's happy therefore he's happy, because all most men want is a quiet and easy life. You insisting on not putting up with being othered, you naturally not being ok with that, makes you the problem in his eyes. And makes him the problem in mine. He might be quiet and well mannered on the surface, the perfect H or whatever you initially referred to him as way back when, but all I see is a grade-A shit happy to throw his wife under the wheels of the mummy-bus, just so he can have a quiet life.
Problem is you've got a DC with him now so you're damned if you do and damned if you don't, you can't fully cut the tie. If you split though you can show both DDs what it's like to be part of a healthy family (assuming you stay away from all toxic batshittery men (or their parents!) in the future). Whereas if you stay you can't. If you stay you can maybe protect DD 2 from some narc time with Grandma but only by throwing DD 1 under the bus and neither DC will be happy living in a home with a half-sister they hate. I see it as DC being part of a healthy family dynamics part-time is better for them than being mired in toxic dynamics 💯 of the time. The DC would hopefully have no reasons to hate each other when with you because you'd treat them fairly/equally and neither would live with you full-time.
I'm also quite easy with these decisions nowadays because I don't want, and most men definitely don't want, to be in a sexless marriage and I just can't have sex with someone who has pissed me off and not resolved it. I was in one abusive relationship and I've got no tolerance for that nonsense at all now. Maybe it's different for you.
IDK if this is helpful because IDK exactly what I'm trying to describe here, but it looks to me like some weird kind of version of the "pick me" dance that you're doing. It's her you're trying to get to pick you though, like she's your spouse, and I'm not sure who, if anyone, is in the "other woman" role. You've got to stop, you're harming yourself, she doesn't want you. She'd be very happy if you split up I reckon, so she can have her DS all to herself. It's kinda creepy. Turning her biological family into her entourage and othering anyone who sees through her, because they're of no use to her so they may as well not exist.