DEAR ABBY: I'm a single mother of a 1-year-old boy, Zack. His father, my ex, has shared custody, but we don't have a very good co-parenting relationship. Shortly after I gave birth, he left me to be with a woman he had met while I was pregnant, and they are still in a relationship.

She thinks it's OK to post pictures of Zack on her social-media sites. It is incredibly hurtful to see another woman pretend she is in a "mother" role with my son.

I have confronted my ex about this and he told me it is none of my business what happens while Zack is in his care.

Am I right to make this an issue, or do I need to move on and accept that this woman is in my son's life?

- Offended in San Jose

DEAR OFFENDED: I sympathize with your feelings, but your ex is right. Unless something is happening that's dangerous to Zack's welfare, you can't control what happens when he's with his father.

I don't know how serious this new relationship is, but it appears to be a long-lasting one - and whether you approve, this woman is in your son's life in the role of part-time mother. For your sake, you need to accept it because the alternative is to make yourself sick over it. And because it's hurtful, stop reading her posts.

DEAR ABBY: I have been invited to a high-school graduation for twin girls. One of them has been our baby-sitter for almost three years, and we have formed a close relationship with her. We know her sister, but do not have the same relationship with her.

We would like to give our baby-sitter more money for graduation, but do not want to offend her sister. What should we do?

- Can't Decide in Ohio

DEAR CAN'T DECIDE: When you give the girls their graduation cards, enclose a check in each for the same amount of money. Later, privately, you can give your baby-sitter something extra as a token of your appreciation. That way, it will be less obvious and may avoid hurt feelings.