I doubt the older girl would remember, she would have been 3 years 3 months old at the time her sister was born, assuming the birth dates are correct. They only thing that would be useful for the prosecution would be if Jaycee was restrained sometime between then and 2009, then potentially one or both of the girls would be old enough to remember that. But I think that we will find that probably didn't happen though, otherwise I'm sure the DA would have brought it up in his opposition to the Garrido's motion.
The sleeping in the same bed thing Garrido mentioned in his interview, was probably metaphoric not literal. There were beds in the tents and none of them looked large enough for multiple people to be sleeping in.
Based on the arguments presented in the most recent hearing, one of the themes the prosecutor will be probably be using is the idea that Garrido was manipulative and implicitly threatening, to support the unlawful imprisonment charge which covers the whole 18 years. I'm guessing that they would use Jaycee's testimony to support that. That is potentially where the girls might be usefull to the defence, depending on what exactly they have to say about it of course, since for at least part of that period they would be able to give witness about what they percieved to be happening.
The prosecution could side step all of that by limiting that charge to the period before the girls were born. If that were to happen then the only testimony the girls could give would be regarding PG's state of mind and behaviour while they were alive.