There are theoretical risks about shared razors and transmission of certain blood borne infections, like HIV and viral hepatitis. It's only theoretical with few if any proved cases of transmission actually happening. For sure this is not a concern for other STDs.
Welcome to the STD forum.
There is no realistic possibility that your mother in law has any current STD consequences from her husband's distant past behavior. If she does (or did), she already knows about them, e.g. HPV, herpes. Otherwise, any STDs she had would have cleared up on their own years and years ago. Or they would have gone away as a result of the antibiotics most people get over their lives for respiratory, skin, urinary tract and other infections. And most philandering husbands do not catch any STDs anyway; and when they do, usually they know it and take measures to protect their wives.
This is an STD information service, not a general family advice service. But if this were Dear Abby, Ask Amy, or Ann Landers, undoubtedly the advice would be that this is none of your business and you should say nothing to your mother-in-law. It's probably a good bet she knows all about her husband's extracurricular activities anyway; all you'll do is open old wounds. And she is responsible for her own health. There truly should be no worry about STD problems 20+ years after the possible exposure time.
Finally, getting STDs from toothbrushes is ridiculous. As the name implies, you have to have sex to catch and transmit them. The CDC certainly does not say toothbrushes are a risk. Someone is pulling your leg or didn't understand what s/he read. If your concern is that you or your family could somehow be at risk by sharing a household or otherwise having contact with your mother-in-law, it is unfounded. There is no risk.
Regards-- HHH, MD
3. I've also read from the CDC that you can get STDs from shared razors (the kind to shave your legs). However, my doctor friend told me that this also is not true; that all STDs die as soon as they are exposed to the air, even if on a razor. Who is right?
The questions about the toothbrush and razor are very relevant to me now because we're trying to get pregnant, and I don't want to have to worry about this anymore. Thank you.