As my suspicion is focused in MGM3 burning the victims body, here is a forensic study of why and how it is done.
https://www.dovepress.com/forensic-investigation-of-burnt-human-remains-peer-reviewed-fulltext-article-RRFMS
MGM3 in this case I suspect is MGM2s sister, may have tried to destroy the DNA. In attempt to relocate the burned body, she may have stopped on the road, to let in fresh air in the car. She may have puked in the car from the smell and the police got the results from the forensics about the DNA belonging to her. In my experience, a severe burned body is easy to detach, especially the feet, so a body part couöd have fallen out while the car was stopped and she didn't notice, ans then continued to drive to Alingsås. Motive for this? She has to save her brother. Motive 2, her son may be involved too, as it is said here that he was in Tishkos apartment before all happened. The police can determine she is dead without finding the rest of the body if the body part was so severely burnt. Stycking is not the case here as it's a lot of work and time, this is a "fix it asap" case.