I'm the RP, I've been separated from dds dad almost 15 years, he cheated, her now stepmum was OW.
I have NEVER objected to her having or displaying ANYTHING from my ex, his 2nd wife and their children. They are dds family too. Heartbreaking for me in the early days but that's not dds problem.
The weird thing is my ex OBJECTS/ED to my dd taking/having anything from home when going to his for contact. Wouldn't let her contact me, refer to me, talk about me...
He's in the military so she was quite a distance away for contact which especially when she was little worried her.
This (and several other issues) are why she won't visit now (she's 16 he hasn't seen her in nearly 5 years as he wouldn't organise his arse properly, he calls once a year).
Parents on whatever side of the divide need to grow the fuck up and do what's best for the child.
That said don't engage him in further subterfuge from your side he has enough crap to deal with by sounds of things. Carry on being a lovely stepmum.
Btw my ex's wife as a stepmum is lovely, SHE has stayed in touch with dd and lets dd know how her half-siblings are doing and speak to them etc