I also recommend a book called The Caregiver's Guide to Dementia. Some cases can be really challenging, but remember, you are dealing with a disease as well as the person who has it.
Yes, +1 to this advice to go to the doctor for a "routine" checkup but alert the doctor ahead of time about these issues. This is what we had to do for my grandma who was very much in denial that she had any real issue and refused help. She trusts her doctor, so my mother dropped off a letter with a kind of 'journal' of the week's symptoms, plus longer standing issues & concerns about safety. In the letter, she requested he perform a quick evaluation and then refer us to a memory clinic for official diagnosis, but to PLEASE be discreet and make it seem like it was his idea. Since my grandma trusts her doctor, we knew she'd at least indulge him even though she was very dismissive of us. Worked like a charm! Got the referral, because he suggested it she went along with it, and we found out she has pretty advanced dementia (not unsurprising to us, symptoms are similar or a touch worse than what you describe). Some of the family including her youngest son is still in denial that it's any cause for concern or that it's even true. Thankfully, my mom has POA and doesn't need his blessing. I'd recommend your SIL look into POA; it's good to have even if your elderly parents aren't currently sick/incapacitated because who knows what could happen on any given day. Family will be in denial for a while, that's just a common thread you'll see on here. As humans, we tend to try to ignore the bad & hope for the best, and furthermore in America (assuming...) we tend to put mental health & cognition in the backseat to physical health. You're doing the right thing by bringing it up. Keep a journal. Read some books for tips to help YOU and your SIL cope (can't recommend this one enough). I know that often, even harder than dealing with the patient is dealing with everyone else denying there is cause for concern. Your concerns sound very valid and very much like dementia to me. Good luck.