Good thought, but I dont think she'd be her usual happy self, as Olivia descibed her, if this was the case.
I just can't believe anything Olivia says and we only have her word for it that her and Allison were close. Judging by her protectiveness of her brother that we have all seen, I wonder if Allison would ever have confided her personal problems to her.
In Olivia's last phone call to Allison she may well have put her happy face on or Olivia could be lieing anyway as the family were pretending that Gerald and Allison had a workable marriage and that they are all a big happy family and all that bullcrap. Allison's depression was probably "self-inflicted" or nothing to do with what she had to put up with her lousy husband according to her in-laws.
I would find it extraordinary if Allison had not suffered depression, anxiety and low self-esteem with what Gerald put her through. I also wouldn't be surprised if Allison had protected Gerald and her marriage in life as well. I get the feeling she was quite private about her problems and gave other reasons for the depression to various people rather than talk about problems with Gerald. e.g. malaria medication, post-natal.