Controversial, but until they are diagnosed with a mental illness, you have to assume they are in control of their actions regardless of their age. If a medically mentally sound 70 year old man is sexually harassing people, he’s just as guilty as someone half their age (not saying your parents would do that, just proving a point).

My personal opinion is you choose who you have relationships with, including family relationships. If you want to keep a relationship with them, you have to accept that they are choosing to act this way (again, assuming no medical/mental conditions) and decide to love them anyway.

If they are actually experiencing mental decline and not just relaxing their inhibitions, as so many older people tend to do, then you need to get them to a doctor fast. Everything from medication side effects to vitamin deficiency can cause mental decline. If they have early dementia, or some other incurable mental ailment, talk to a therapist about how to handle the situation and stay sane yourself. Caring for someone you know will never get well is extremely hard emotionally.