This is up to the father to fix. He must be adamant about how family is in his house, disrespect will not be tolerated. BM is coaching her. Treat her as such and shut it down now.  Also focus on all the things/freedoms she has at her compared to helpless newborn.  BTDT.  

I agree with with Survingstephell.  It sounds like BM is facilitating this...HUGS

Who cares. Just ignore her brattiness and only address things if she is a danger to the baby. Don't let her steal your joy as a mother. This is about you and your baby, and if SD has a problem with it, your DH needs to shut it down and not cater to any BS.

Okay, but he's her half brother because they share a dad, not because you are dad's wife and stepmom. That might help her understand better?

It is normal for siblings to be jealous of new siblings. I adored my sister our entire lives, but I did not want her and I was devastated when she was born. Things might turn out okay with her, just keep talking to her about it and know her feelings are normal.

I never had a child with DH (thankgod!) because it would frighten me our baby wouldnt be safe from SS13.  However SS13's mother had another baby with her new husband and she now has another on the way ,, and oh geez has she copped some crap for her brat son !! he HATES the baby and HATES the fact mom is due to have another in july.  I feel sorry for the stepfather in this case as hes actually a really decent guy and cops all the crap i was put through with the brat! Even though i despise the BM , i actually worry that this kid would be vindictive enough and certainly wouldnt trust him alone with the baby .  Your DH needs to be firm with SD but also include her with things 

I would not necessarily classify this as SD-10 rejecting her STB born little brother.

Certainly BM could be in her head on the whole thing, and likely is.  The usual "the baby will replace you in your daddy's heart", "SM will not like you as much as the new baby", etc..... etc...... etc....... is most likely.  The counter to that kind of crap is daddy not tolerating any crap, giving the SD all of the facts that counter BM's toxicity, and ..... you partnering in all of that with DH.  

IMHO DH needs to start engaging her on the excitement of a new baby and a brother. You being her SM has nothing to do with the baby being her half brother, they are both DH's children.  A bit more explanation from DH might start clearing things up for her and start her in engaging on the family excitement. If not, then cutting her PAS'd  kid bullshit off in its tracks is a second best outcome.

IMHO of course.

We have seen regularly on st, plenty of skids and half siblings on the newborn baby etc, have decided early on they want to remain in the marital home and pretend we all don't exist and instead of being positive role models towards younger siblings, they'd rather be selfish arseholes!!

my ss didn't acknowledge our daughter for 5 weeks until hubby confronted him about it. He wanted to come to hospital the day bubs was born but hubby said now. Even as a 17.5 yr old, hubby knew that when he'd come, he'd look for a second and start shouting at hubby rudely demanding to be taken home and dig his head in his hands and fidget/scratch his head out of boredom. My ils came as were all so excited as this was the first baby in family in about 5-6 yrs. 

the next day ss come and as predicted dug his head in his hand. I gave birth in malaysia, i'm caucasian and there were 2 babies born that day, a brown asian malay one and a white one. Ss17.5 asked daddy which one was his. My husband looked at his kid like the idiot he was and said the white one, like seriously what kind of stupid question was that.

when our daughter was 5 weeks old, he told his dad he was incapable of showing any love and to not force him to and this was in response to his dad saying it was disappointing that he pretended she didn't exist for a whole bloody month. But when ils are over oh boy does he fake the attention with awkward strokes on my daughters hands and she looks at him like "get the eff away from me!!" And the ils call him out because to them we are family and they can't understand how someone who is closer related (more than the ils won't accept our kids)

i have a daughter and son and the relationship they have together and with their dad sets the bar and example for skids.

sd's are 26 & 15.5 and went mia for over 5 yrs and expected daddy to be pining for them and they guilted him for moving on with life. You know what, he found someone he loved and wanted a family with her, a real family and not the fake made up shit they have thats barely a functioning relationship.
 

They would refer to us a "the new family" and i kid you not after 5 yrs mia and a nasty arse divorce from batshit effin crazy bio mum, they proceeded to lecture daddy to maintain child support indefinitely, that bio mums marriage to her affair husband might be ending (not that my husband gives a flying shit about this) oh and could you transfer the home you bought for us to live in into skids name only because bio mum doesn't want stepdad to claim it if they divorce. A home hubby bought after the divorce on his own with no papers signed by bio mum. 
 

hubby refused to and said he has a loan on it still to be paid off. I'm lucky hubby actually offloaded a whole chunk of early retirement savings to buy a home in my country of birth thats solely in my name to avoid batshit crazy and co seeking to make us homeless

its been almost 2.5 yrs i have not been in contact yet alone met the 3 ferals. Just wait for the day when the ils start saying to my kids aboht skids thats you brother and sister and my kids say "no they're not" because really they aren't by nature so lets not glorify them just because of the small dna they share, the same amount they share with their aunts who treat and love them as real family.

there will never be a day i will ever want anything to do with any skid. That bridge they burnt a long time ago and no sob story or guilting by my husband will change that