I don't think parents realize how weird it feels when someone tries to "force" you to "love their child". It's awkward and very....unsettling.

I can say, as BM and SM of teen girls that arrangement won't work down the road. 

We classify them as stepsisters. But they are not related. Which leaves a need for privacy, very different then with actual sisters raised together. Not to mention the huge age gap adding more issues. 

This came up in our home, BD sharing with SD. My BD sat us down and explained that she related to SD as a friend and felt that her need for privacy should be on that level. She wouldnt dress in front of a friend so why are we expecting her to dress in front of SD.

That has always stuck with me. And it took a kid to explain it

Do you have a shortage of bedrooms such that they have to share, or does your wife just want them to share?  5 years is a big age difference, and it's going to seem bigger before it seems smaller (8 and 13 will be miserable!).  At 3, your daughter is going to be getting into the 8 year old's stuff, which could lead to fighting and resentment.  They'll probably have different decorating taste and different needs for what is in their room.  I think you can approach this from an angle other than your desire to do things with your daughter without the SD there.  

I wont' do it. My son has a very small room -barely room for a bed and dresser and a few toys. When SS8 is over he sleeps on a blow up mattress in the dining room. BM says we must allow them to share a room but I will not do that. For one - SS8 is exposed to a lot more than I want my son exposed to, and he is unpredictable - I don't trust him alone with my son. Sad but true. Secondly - BS is just like me and sometimes needs his 'alone' time and his room is where he goes for that on SS weekends. He deserves that as much as I do. When SS8 is over he knows the two bedrooms are OFF LIMITS!! That is one hard rule in my house and I do say 'my' house because I pay all the rent and bills in the place!

Is there a reason you don't feel comfortable inclusing yout step-daughter?  I know you said that she's clingy and can whine wtc, but why not show her that she has value and include her?  My biokids and I use to have a reoutine of prayers and some songs etc, and we introduced them to my stepchildren.  They love it, and it's something they've continued to do together despite me no longer participating because they're getting older.  Try to be empathetic and consider things from the child's point of view and see if that helps.

I am six years the elder and my younger brother and I shared a room from ages 10&4 until 14&8.  It worked fine. But... we were not allowed to fight and neither of us was allowed to interfere in the other's turn to be the age that we each were. 

When we moved and had a home where we each had our own room it was an adjustment. There were several instances over the next 8-ish years that one or the other of us would go to the other's room in the middle of the night, pull a huge bean bag (mom made each of us one) next to the other's bed and crash.  We remain close to this day and I am 54 and he just turned 48 two weeks ago. 

We call each other regularly and whenever one or the other of us is traveling for business and can get no more than about an 8hr flight from each other we make sure to meet for bro time.

The Skid/BK dynamic throws another level of complexity into the room sharing dynamic and the things that you outlined are totally legitimate and should drive the decision.  What is the primary motivator your DW has for wanting the girls to share a room?  Is there a space issue?  Is she wanting to turn the vacated bedroom into a design studio? ????

Before we shared a room mom and dad would come into each of our rooms and read, tuck us in, and otherwise have some focused time with each of us. When we shared a room they still came in.  And when we again had our own rooms mom and dad kept doing what they did.

The dedicated 1:1 time and privacy issue is important in a yours/mine Skid situation and can and should be respected while integrating the two families effectively.  If the bedroom availability will allow.  You can also tuck in your SKid and read a story to her in StepDad/SD 1:1 time. This may relieve your DW's worries.  I used to do this nearly every night with my SS-25 when he was young (from 2yo until the early double digits).  I  have no BKs so the dynamic was not as complex as the one you have but it worked for us.

IMHO of course.

Good luck.

My husband could have written your post. He's the exact same way, pretty sure he feels that way but won't say it outloud. Doesn't need to bc his actions speak clearly and I've grown to hate him for it. It's pretty much saying that your child is worth the investment and love but hers isn't which in turn she has to fill the gap to overcompensate what you have withheld. I understand yours and my own husbands feelings but the child doesn't. My kid is older than yours and let me tell you, you keep that up and you will have MAJOR issues with her as she ages. It's not going to be pretty and it's going to ripple out into your marriage unless you both have a marriage of steel. The room sharing thing, no wouldn't do it just bc girls eventually need their own spaces period. But it makes no difference their having seperate rooms. Her daughter will still notice every single thing you do. 

I have an 8 year old SD and have a 3 year old BD. They share a room and have for about a year but they won’t in our new home we are about to build. I will tell you my SD loves having me tuck her in and read her stories too. We do it all together. Our situation is 50/50 however so I do get alone time with my BD when my SD is with her mom. I do appreciate those moments alone so I can see where you are coming from but as a STep parent I’ve learned to eat up what I feel I deserve sometimes and do what’s best for both kids. They both want and deserve your attention. It’s easy now bc my husband and I both don’t have to split time between kids to read. They equally get both of our attention at night but I know that set up won’t work forever so as far as down the road they will need their own rooms. The age gap alone is too big and not fair to either child. I had to make that decision as well and my husband agreed because of the age gap. 

yeah I agree with some of the other commenters - that the tucking in and bedtime story thing - why can't you do it for both kids? My son loves it when my BF reads him stories at night. It has really helped make them closer. Also what is this telling your BD? That is her (half) sister - I'm sure she loves her (though I'm sure they squabble). If she sees that you treat her sister as less that will either set her up to be entitled and bratty and treat her sister poorly or it will make her resent you. Either way, not a good outcome. 

Unless they defo need to share a room though  because of money - they shouldn't be - different bed times - needing space, etc. with that age gap. Although it's probably just ok for now.