I got pregnant pretty easily the first time, but miscarried and then nothing. Your SIL should have an infertility workup. At the very least she can have a semen analysis and basic hormone check with her regular doc. Though she should not waste her time with any treatment with her doc, she should see a reproductive endocrinologist rather than a regular OB for anything beyond basic diagnostics. Long cycles are definitely a concern. As a pp said she should also be charting her cycles. It can be a pain, but worthwhile and a lot cheaper than ov predictor kits. I realized from mine that although I had very regular cycles, I was not ovulating. Once I got my thyroid regulated I was ovulating, but still not conceiving.
I definitely recommend seeing an acupuncturist skilled in infertility. If your SIL is at all open to it, a great book for the Chinese Traditional Medicine approach to infertility is The Infertility Cure, by Randine Lewis. You can sort of self-diagnose your issues and make dietary changes and stuff to help address the underlying issues.
A lot of docs will just automatically give you clomid for several monthss when you are having issues, and if it ends up being "unexplained infertility" they have their list that follows. That was me, I had no particular medical reason for not conceiving. That approach was too one-sized for me so I did acupuncture, took herbs and made dietary changes to get the underlying issues addressed and then talked my doc into trying unmedicated IUIs for 3 cycles so I could be certain that we weren't just missing the window. That worked for me and I swear it was the acupuncture and herbs that were responsible. I had my baby 3 years after my miscarriage.
It is so nice of you to be inquiring on your SIL's behalf. Infertility is extremely stressful and painful. People say a lot of unpleasant things that make it difficult socially. Even worse, it becomes absolutely obsessive because you are constantly on a clock for something. Every few weeks you mourn when you get your period then a few days later you are upbeat because you have another chance this month. It's positively draining. It is nice that she has you there for support. She should also find infertility support, there are some good boards out there.