I've been wondering about other unsolved homicides or missing persons (especially young girls) for the areas that Nichols lived during his lifetime. It is hard to believe that this double murder would have been a one time occurrence, unless this was an "escalation" type occurrence. It would be interesting to know where he went on these construction jobs, etc. and where he visited other family. Any clues from the family would certainly aid in the process of researching this.
You mentioned (I believe) that Nichols had a criminal history? Does the family believe that there may be other cases involved? You also mentioned that on the morning following the night the girls went missing, Nichols had scratches on his face and said he had seen the girls at the beach. The family also relayed to you that Nichols mentioned "something" that had not been publicized that he would not have known had he not been in the area. I don't blame you for not posting this for fear of damaging any subsequent police investigation. The fact that he left Carlsbad right away when he was supposed to have spent the remainder of a week there is alarming. Didn't you say he disposed of the car shortly thereafter?
It sounds like Nichols' family has had a cross to bear for many years in keeping their suspicions to themselves. They should be commended for coming forward, even if nothing comes from it. It sounds like they are trying to make "peace" before death, especially in the case of the elderly daughter of Nichols - and that the granddaughter is helping with the process.
This tip sounds to me as if it really has potential. Although detail was not given about the brutality of Patty's death (nor does it need to be), many were lead to believe it was a personal vindictive attack. In my opinion, it sounds more like a cold, impersonal attack of a very sick individual. This is how the perp got his "kicks," and I wouldn't be surprised if links to other crimes are discovered.
It seems more unlikely to me that the other suspect would be involved, especially since he could have easily made arrangements to meet Patty without Mattie being there. If they were having relations, forcible rape would not have been necessary. The Blair connection, however, is an interesting one, especially with the father finding the bodies - maybe too much of a coincidence - time will tell. It could be possible that Blair disposed of his car because Patty had been in the car at some point and he was afraid of being accused of committing the crime if evidence was found that she had been there.
Law enforcement should be orchestrating DNA tests and gathering statements - and very soon! I would think this would be their responsibility - cold case or not. This is like a "dream tip" and should be treated very seriously.
Sunshine1950, I visited the new website which was established for the girls. My heart fell to my stomach when I opened it and saw the two beautiful faces of Mattie and Patty. It truly becomes personal when you put faces on crimes. I believe you are getting closer and closer to finding some, if not all, of the answers you are looking for --- and that law enforcement will soon step up to the plate to assist you. In the meantime, we'll all keep websleuthing to help them along since they seem to be lacking in motivation to get this case solved.