Was there no DNA evidence on Somers' body? Or, have LE just not released the results?
There must be some if they think she scratched the person, but they have not said anything about it. If it's not a match for a known person there really isn't much to say about it.
Why does mom have a lawyer?
Because it makes absolute sense. She's been questioned by LE in relation to a crime. A sensible person wants an attorney for that. She knows whether she is innocent of any involvement, but that doesn't mean that she can convince others of that all alone. What if someone associated with her did it and tries to frame her by planting evidence, etc? Having an attorney is not the same as failing to cooperate. Nor is it an indication of guilt or innocence.
Plus, there are all sorts of issues she would need advice on. I'm sure she wants to know what evidence she is entitled to see. Someone in her home could have been involved. Others might be making libelous statements about her. There would be issues related to the donations she's taking in.
In addition, she seemed to be sort of running at the mouth in the press. She doesn't want to say something now that could be damaging to her testimony later. (Think of the debate people are already having over whether she even knew what Somer had on that day. The talk of the people she associated with, etc.) If they find the perp, she may want to sue him at some point.
Most importantly, she can discuss all of this stuff with a lawyer, and it can't be disclosed. Not so with victim's advocates, family, PR people, etc.