One of my kids turned 18. As soon as that happened the pictures of the parents disappeard. Is that normal?

Yes, the lineage remains only until they turn 18.

Yeah I had set one of my male villagers for parenting and forgot I had him on that and before I knew what was happening some of his daughters had grown up and he was going to visit the love shack with them and now they have kids by their dad... At least it's just a game thank goodness...

aww, why?
I gave them last intials. I started with A. Girls will take the last initial of what ever guy she "mates" with. They will not be allowed to inter-mingle with people of the same letter.

I believe the idea is that the parents are there while they're still "in the family", before they're "out on their own". Just like North American society, where a person usually stays with their family until about age 18 before moving into their own apartment. The villagers are only nursed until age 2, but they are still kids under their parent's watchful eye (imaginarily, since the parents don't actually watch their children, only you do), until age 18. Something like that.

I think the idea is more that they'll still inbreed, and they took away lineages at 18 so maybe we wouldn't notice/it wouldn't bother us (especially when we're in those tight spots.)

I have found it very difficult to grow my village population without in-breeding. I kept a spreadsheet of them, who their parents were, etc. but it became very time-consuming and it was often difficult to find suitable partners for villagers, there were a lot of kissing cousins! Now I've decided that in-breeding is okay on Isola because it's a little paradise where real life rules don't have to apply

Barbara, My children display parents up to age 15. When they turn 16 my parents disappear. Does mine have a glitch?

There is a small glitch in the display of parents that will be fixed in the update. They are supposed to display until age 18.

i would like it if they would stay until the day they die, because i like to keep the same couples together. i don't like to make my villagers cheat on each other.sorry