Dear IrregularFamily: As you have surmised, such very irregular periods usually reflect very infrequent ovulations--and infrequent ovulations decrease fertility. That your sister spent a year on Clomid (ovulation induction medication) before conceiving is another bit of evidence that ovulation is a part of the problem.
I really doubt there is a vitamin deficiency that is the culprit. You mentioned possible PCOS (notable for its lack of regular ovulations). Currently a woman could be considered to have PCOS is she had
any two of these characteristics:
1. Irregular, or absent periods due to missed ovulations
2. Increased male hormone levels (e.g., androgens like testosterone)
3. Multiple small ovarian cysts ("string of pearls") visualized on pelvic ultrasound
These are the Rotterdam ESHRE/ASRM (2003) criteria for diagnosing PCOS, and as you can note, body weight is not part of the diagnosis of PCOS. Up to 70% of women with PCOS are obese. Some women with PCOS are lean.
Costa (2010) compared 35 lean PCOS women to 162 lean non-PCOS women and 28 overweight PCOS women to 58 overweight non-PCOS women. That study found increased risks for heart disease in thin women with PCOS. Even being slim, women with PCOS still had elevated triglycerides and increased waist size ("apple shape") compared to non-PCOS women of similar lean weight. So it is possible to be thin and still have PCOS.
There is a genetic predisposition to PCOS. There is even a genetically linked male version. Thus your irregular periods might be genetically linked--especially given your sisters' histories.
There may also be another genetically linked condition which is influencing your ovulation patterns. At age 32, and wanting to conceive, I would strongly urge you to see a reproductive endocrinologist or infertility MD for a consultation. You need a more "for sure" diagnosis before considering treatments. Truly, it sounds to me like you are doing everything right to be naturally fertile.
In Support,
Jane