I'm worried about big sis, she is forgetting things. how do i suggest testing for dementia?

Be caring: In a gentle, firm manner start with why you are concerned.Then discuss with her what the two of you could do next.

Many causes: Your sis may have hormonal or vitamin deficiency contributing to her memory problem. Try her on a basic good quality multivitamin to start, along with some coconut oil. You can buy coconut oil in gelcaps, or buy it in bulk and use it liberally with cooking. You can get coconut oil unflavored. If no improvement within a few wks, seek attention for further evaluation.

Enlist support: If memory loss has been present for some time and isn't just an isolated incident, i agree it should be evaluated. How to go about it depends on the relationship. You might be able to suggest it directly or enlist the support of a spouse, children or other family member. I've also seen family members suggest you both be evaluated.