Miss Blake and her children were last seen in east London on 13 December, prompting a missing person investigation, but the murder squad did not get involved until 4 January.
Her sister has revealed the former actress was planning to leave Mr Simpson-Kent over Christmas and wanted to end the relationship "a long time ago".
"She had asked to come back home and we said yes, so we really tried to plan on getting her to move back home," said Ava Blake.
"She said she would come back, sort of Christmas week, and after Christmas they would arrange about selling the property she lived in.
"She didn't want to throw (Arthur) out on the street but give him time to find somewhere else to live.
"That was the only point she was wavering on."
Ava Blake said her sister had changed from the "vivacious, happy person she once was" and that she suspected texts sent by Sian after she disappeared had not been written by her.
"The first part of the text, I believe my sister may have written that," she said.
"But the rest of the wording and other texts my family received, they were not my sister. We don't use text lingo. We write full sentences."
http://news.sky.com/story/1618724/sian-blake-died-from-head-and-neck-injuries
Im guessing its a argument that got violent, but why the kids? thats the bit I just dont get.