How to tell DD about estranged half sibling

[Kate1556]

I’m having trouble deciding what to say and when to tell my 6 yo DD about her half sibling.
For background, my husband was married for 3 months prior to us meeting, in this time he was cheated on and emotionally abused. He married his ex as she told him her child was his (he was not) however the marriage produced another child.
After the volatile relationship ended he found out she was pregnant however has only met the child a few times in the 10yrs since.
I don’t want to keep this a secret from my DD but to fully explain the situation there are a lot of adult themes, I don’t want to gloss over the emotional trauma her daddy experienced but feel she is too young to deal with it.
The half sibling has a large family of her own and has never asked to see her biological dad or our DD.
Help!!

How old is your DD? Can you just explain that her DF was in a relationship before he met you and had a child? There is no reason she should need to know about the trauma her dad went through in his previous relationship, not her burden to carry.

Oo sorry just seen she's 6.

What efforts has your husband made to see his other child? Saying a ten year old hasn't asked to see him is v odd- how would you know?

Your story is very focused on his ex and your husband's distress, your daughter will only be interested in her sister/brother- more so as she gets older. I think it's important to think about what you're prepared to do to facilitate that relationship.

It’s a bit complicated but I know one of the family members of the child, my husband has only seen her when she has been at this family members house.
His Ex has a large family with different partners, none keep in regular contact with their dads which has become the mantra of the family.
He cut ties to end the emotional manipulation and abuse which is why we haven’t been involved. I think for my DD to understand the situation she needs to know how it happened, to his Ex he was a sperm donor which is why she has made a relationship extremely complicated

I don't understand why your DD would need to know that his ex treated him like a sperm donor?