Tell her you don't need or expect her help with the baby, she is not its parent. That's clearly one of her normal comments actually. As for bedsharing, tell DH that it's inappropriate for a 13YO to bedshare even when awake with adults and that you can't be in an inappropriate setting any longer. Send him to the LR to lie on a sofa with her. If that ultimately creeps you out, tell him to get therapy himself.

One has to ask why did you allow yourself to get pregnant? Did you think it would distract him from his first born?

Is TV really that important to you? Don't watch it when she's around. Record anything you would really miss. Get a recorder if you don't have one.

I'm going to asssume she is a weekend visit. Abandon your husband for those two days and let him do what he wants with his kid. After all that is the point of her visiting - to be with Dad - not you.

Visit Mom, read books, visit a girlfriend or the museum. Stop interacting with the girl and let them do their thing. Don't comment, don't argue just don't get involved.

Use the search engine on this site and enter 'disengage' and read some of the Posts on the subject. This is what I did:

I wrote this a long time ago and I hope this helps with your problem. It may not reflect your exact situation but you'll get the idea:
The situation with my step-kids finally got to the point where I decided that my efforts to raise them was futile.
First I told my wife in private what I was going to do and why.
Then I stopped investing myself, time and money in their upbringing. I realized that by conscious choice I could cease being responsible for them or their actions. Neither would I take credit for how they turned out, good or bad. I simply stopped interfering in their lives. This is not to say if they would start a fire on the living room floor I would not intervene. But if they did not do the laundry as scheduled I ignored it. Nor did I do any of their chores. If the trash overflowed in the kitchen - well tough. Mom dealt with it when she got home.
You will be absolutely amazed at the look on a kids face when s/he realizes you don't care enough to even tell their bio-parent they did something wrong. And you'll be amazed at their future behavoir and attitude towards you.
I stayed friendly enough and taught one how to drive when she was old enough. But they could no longer blame me for the consequences of misbehaving just because I saw the misbehavoir. I never told. If they were caught in a infraction it was not my fault. They began to realize that they were responsible for their actions because of what they did not because I caught them.
I would talk to my wife in private if I had issues, but once she made the decision I backed her up 100%.
Once the kids realized I was no longer the ogre they thought I was their attitude changed. I was not their friend, but neither was I the enemy. They came to realize that I had not done those things to irritate them. They in fact missed the things that they had come to depend on me doing. Permission to go to the mall when their Mom was not home? "Sorry, can't do that".
Their mother slowly came to realize that I wasn't overreacting to their actions. In fact I wasn't reacting at all. She came to understand that she would have to control the situation and she did.
With that things got much easier around the house.
Billions of kids grew up in the world without help from me and turned out just fine.

I'm not understanding I guess...
Is your SD visiting every other weekend and wanting to spend time with you both by hanging out in bed with you guys watching TV in the evenings? Your husband doesn't expect the 3 of you to sleep together, right?

If she lives with you both full-time then I think your idea of moving to the sofa in the living room, for example, is not unreasonable. However,if she's only there for 4 days out of the month, why are you so upset? Sounds like this child is just trying to be close and it grosses you out. 13 is still mentally and emotionally very young.

Baby talk is sickening at any age- accept baby age. Encourage her to communicate using her normal voice and tell her to save her baby talk for the baby. Tell her that you won't respond to her or give her what she wants until she complies.

You're bringing a new baby in - her sibling. You want to be able to trust her with this baby. Don't push her out. Find a way to make a snuggly space for all of you to watch TV together if you just can't stomach having her in bed with you.

Or - maybe I don't see the whole picture and this kid needs a psychiatrist. But something tells me that if she feels secure and has a place in your home that within a year she is going to be more interested in hanging out with her friends than spending the evenings watching TV with you 'Lame-Os". But, if she doesn't have a place in your home she will feel she is being replaced by this baby. And think about how that will suck for her. And think about how your life will suck then.

I went through this with my SD when she was much younger around 6-8 yrs old. She is 14 now and still tries to compete with me for her father's attention. Alot of her wanting him to sleep with her and the attention when she was younger was something her BM put her up to. I explained to my husband that she slept alone at BM's and she could sleep alone at our home.

This is the time when you need to establish yourself as his mate and a parent. She doesn't know her place and she's doing everything to get attention. Point out her Babytalk and ask if she talks and tell her it isn't acceptable.

You also need a TV in a common room. If it's evening she doesn't want to be left alone until bedtime.