I remember this. In 1990 I was the deputy sheriff for that area. I remember your mom she was a cook at a resturaunt right?
I was off and out of town the night this happened, and as such I did not have anything to do with the investigation. As I remember the woman that did it pled out to a prison term. Here are a few suggestions, but I think all of them are gonna involve a trip to Bay City:
1. Call the Matagorda County District Attorney's office at 979/244-7657. They may let you look at the case file.
2. The Bay City Public Library used to keep past issues of The Bay City Tribune on microfilm. I don't know if they still do. The Bay City Tribune still exists, they may have records also. The number to the tribune is 979/245-5555.
3. Call The Matagorda Sheriff's Office at 979/245-5526. James Mitchell is the Sheriff, and I think in 1990 he was either a day deputy or an investigator. Either way if you explain what you want to him he may be able to help you. I do think that one of the officers that investigated this has passed on, and there has been a lot of personell changes in the 13 years since this happened. I'm not sure that anyone that actually worked on this case is still there.
Good luck. I did not know your mother well, but she was nice to me the few times I talked to her.