He's only been in her life for 2 years so you need to be firm, I wouldn't say about sexual abuse or anything but maybe just point out that it is DD's room and she needs privacy, he has no need to go into her room when she is in there.
Either that or stick a whacking great sign on the door (as I did at that age despite being no where near puberty) saying DD's room, knock before entering!!! In bloody huge writing
He needs to respect her privacy. Lack of respect for my privacy was one of the key reasons for my difficult relationship with my parents as a teenager. It really isn't that difficult to knock, not barge into the bathroom, not open mail (sorry, these were all things my parents did!). It's so hard to be a teenage girl and as she gets older she needs to know you respect her and hopefully she will respect you both. I think you should just speak to your DH.
'DH, you are making DD very uncomfortable when you go into her room without knocking. If you do that when she's getting un/dressed and she tells a teacher/friend/responsible adult about it you could be accused of child sex crimes.'
If he has any sense at all he should get the message, if he doesn't...start to worry.
He needs to be made aware that whether he likes it or not, she is growing and she will needs her privacy. Whilst we still see them as our little babies, I remember as a kid my mum making me try on clothes in the shop and being really embarrassed, I have to remember this now with dd1 and respect that she is showing awareness of her body.
is this about him being "master of his castle"? My dad would never knock in his own house! If so you could try scaring him with the idea that he's leaving himself open to allegations of abuse if he doesn't respect her need for privacy. Personally I found that my girls started the physical changes young, but didn't crave privacy until a few years later.
If he only joined your family when she was 6, I don't think he should ever have gone into her room without knocking, though I realise that's not very helpful now. What does he say when you ask him to knock?
My older daughter is 8, nearly 9 and has just started puberty. (Just breast buds and mood changes so far) my problem is that my husband-not her bio dad- doesn't seem to accept that she's growing up. He's been part of our family since she was 6 and still sees her as a little girl. The main problem I've had is him going into her room without knocking first. I call out when I'm entering, and I've asked my other daughter aged 6 to do the same. I've also asked my husband to do this, but he doesn't. How do I discuss it with him?