Lots of people mentioning here that it must be DNA ruling out attack by someone sisters knew. Well, this is Wembley, Borough of Brent, one of the poorest in the London capital and one with the most death of Covid in the city. While police is great, there is no way DNA was back in 2 days - in Wembley.
I personally lean on that police suspect it was stranger because nobody who attended that birthday party (of those who they know of) had any visible injuries. Frenzied knife attack would leave injuries on suspect's hands even if other party did not fight back. As there was blood trail leading to an exit of park injuries might have been more severe, possibly from other places on the suspect's body as well. (Remember those frenzied knife attackers who accidently slit/stabbed themselves too? Happened on few occasions).
I am waiting to hear on reports if it was few stab wounds or big amount like we saw in some very rare cases...
Someone mentioned here that possible motive could be racial attack. Bear in mind that about 80% of Wembley's population is black, so I personally do not think this was the motive in this case.
Party started around 7pm. As this was the birthday party, very likely some amount of alcohol was involved. Some food, maybe cake and also knife/s to cut it.
Some people were probably arriving at different times and also leaving at different times. Most of the people would be probably leaving park just before it was completly dark. What time is it getting dark now here? 10ish - 11pm? It seems last known people left the party just before midnight, 5 long hours after party first started.
Also, if you ever hosted picknick or other parties, there is always some guest/s which stays behind, helps you clean up, pack up and than walks out together with you. Even more likely happening if it is your own birthday party.
I also personally don't believe two women would stay behind, alone, after everyone left, in any of the London's parks. After midnight? In Wembley? No way.
With regards to blood trail leading to an exit of the park 5 min away. Again, this was not done by DNA but simply ruled out to be theirs due to position of their bodies where they were both found in the park. They never made it to the exit.
Does this all above rule out for you someone who was at some point at that party - invited or not, drinking for 3 to 5 hours? I dont mind what news headlines say, but so far it is still no for me. Or police did not find all who attended or someone somewhere in those statements might be telling porky pies, covering for someone.
(All above just in my humble opinion and subject to change)