It sounds to me as if you have a choice between having some kind of ongoing relationship with your sister or an apology, not both.
She's defensive about her DH, and, having been put into a position where she was running between you on the issue of the weird fortnight's lag in telling you you'd lost the buyer, ultimately chose to defend him. You may be hurt by that, but that's where you are. It's perfectly possible she has glimpses of understanding that she's married to someone awful, but is choosing to go with the sunk cost fallacy and stay married, and doesn't want to hear her sister say negative things about him. It's easier for her to think you're just being a meanie for never visiting than for her to think you seldom visit because you loathe her husband on the understandable grounds that he's a dreadful person.
I also think that, especially if the original altercation was all happening over WhatsApp, that you're possibly blaming her DH for things that were actually her fault. Was the lie about the reason for you not being told the buyer hers or her DH's for instance? Was the failure to pass on the information sooner hers or her DH's?
Basically, though, I think she's been very clear that she's not prepared to discuss the altercation. I think you either accept that and move forward because you want an ongoing relationship with her, even if it's not the one you wanted, or you insist on discussing the past, with the risk that she just disengages.
Was the flat actually sold in the end, or is it ongoing?