As long as your daugther is a minor it is your parental duty to protect her from shit people. Even when those shit people are her elder sisters.  Fortunately, those shit sisters are not full time polluters of DD's life, and yours.

Purge them, prepare your 5yo to protect herself from them as she is growing up, and sadly, once she is an adult. The shit sisters will not in any liklihood figure out how to be decent people.

Take care of your little one, and yourself.

I have done that very thing with my youngest bio.  She has a relationship with my older bios and YSD is getting iffy.  She's 24 now and has her own struggles.  My YBD is now 15 and can keep in touch and does.  She has nothing to do with the other 3 skids.  It's ok and she doesn't ask about them.   She knows they exist.  Drama is best avoided.  If those half siblings bring the drama, feel free as a parent to prevent that drama.  

Honestly, I wouldn't see a 5 year old and almost "adult" girls having a super close sibling relationship... They aren't playmate ages.. they aren't growing up together.. you have a set of girls that are pretty much out the door when it comes to being part of the daily family mode.

Your daughter doesn't have to be shielded from them unless they are doing dangerous behaviors around her.. or unless they are outright abusive to her. btw.. "leave us alone" is not necessarily abusive.. they arn't obligated to want to entertain a 5 yo.. and while it might hurt your DD's feelings that her older half siblings don't want to play with her.. they also can learn the lesson that not everyone is obligated to play with them.. and that other people have the autonomy to want their own space.  Not a bad lesson to learn at home honestly.

So, how do you deal with the questions, if they arise, of where are sis1 and 2?  Oh.. honey they are big girls and they are doing things with their friends.. "they are busy with school, work etc.."  "I don't know.. I'm sure they will come around at some point."    If your child is smart like most kids.. she will get it.. and not hold out endless hope for them to come around.. like she is doing now.

Not all family is "close"  some family is difficult.. its kind of a box of chocolates thing.. you get a bit of everything.. even stuff you don't like.. you can't pick your relatives.. but you can choose who you spend your time and energy on.

I'd let this play out naturally.OTOH, when they DO come around, make it clear your daughter is not a doll and that you, and only you, make decisions for her. We had a situation between cousins withe the same age difference where the older one decided to drive the younger to the funfair without asking the child's mother (not even informing her). When the child's mother realised her daughter was missing all hell broke loose. The older cousin's mother knew what had happened and all she had to say was that there was no problem...  her daughter was a good driver. She couldn't fathom why her SIL was livid.

At a family meal, the same cousin who had my daughters aged 5 playing in her bedroom thought it would be a good idea to let them meet her friends and snuck them out. The friends all had scooters, it was 2 floors down outside, 9pm and dark. DH decided it was time to leave before I did or said something irreparable to his family.

My step-siblings are much younger than I, the youngest is 17 years younger.  We don't have any dissension but we aren't close, either.  The other posters are correct that Its just nature.

Your BD knows they exist, I'd just let it ride unless they do something harmful to your BD.

Looking back, I guess my younger sister and brothers might have wanted more of a relationship with me but honestly, I never gave them a thought.  I was too busy with my own life.

IMHO, the goal is to prevent the elder toxisibs from doing anything harmful. Waiting until they do, is waiting too long.

Put them under YOUR hairy eyeball and make it clear that there will be zero say from them in shit for anything and the elder toxisibs will live hell on wheels if they cause any drama or harm with their notably younger sibs.

I am 6 years the elder to my brother, and was 8 years the elder for my youngerst brother. My parents were clear, I had the the unavoidable duty to protect and care for them in my role as the big bro.  I made a couple of mistakes in that duty. I never made either of those mistakes again.  The disappointment I had in myself made it crystal clear that I had failed in my big bro duties.

The first was with the baby when I made him cereal, cut up some oranges and added them, to the cereal, and he choked on the stringy stuff. I did not cut the oranges into small enough chuncks.  I scared the shit out of myself. I fixed it.  I shared with mom and dad, they comforted me but I knew I had not done my due diligence in the big bro arena.

The second, I was in my early teens.  My brother was about 7.  Mom and dad were away on an erand so I was in charge.  My brother and were tossing stick pin darts at each other alternating running the down the hall trying to avoid the darts coming from behind.  They were not dangerous.  We made them ourselves.  Light, had thread feathers, thrown my hand no blown from a straw.  The look on our parents face send in my direction was all that was necessary to know I had failed my big bro responsibilities.

Of course it is impossible to completely insulate a young kid from the bad choices of an older sib, even when that bad choice is entirely innocent, but, that should be the parental goal.  Particularly when an elder sib is a toxisib.

IMHO.