Amy is tied to Susan events, go to beach in the evening, most days you'll meet them there. Be sure to save before making a suggestion, as you can't scroll back after the choice is done.
In the beginning, buy Ice Ice cream for the girls till Amy has 3 love points I think..
Be sure to have bought Sunscreen and after sun screen, maybe even the third bottle, at the local store before suggesting sun screen.
Amy won't accept, but Susan will. This leads to an event a Susan's home.. not an achievement, but to get the ball rolling.
Swimming a few times eventually increases Amy's corruption.
Can't recall the exact steps, but the last one is invite them on the boat.. that's where Amy's achievement unlocks, so always safe before suggesting to do something, so you can reload if Amy doesn't get points or nothing new happens.
For Maria.. I think you'll have to complete all the other achievements first and then visit her in her room to talk.
If she still doesn't say anything new, visit her in the Church cellar at night a couple times and try during the morning/day in her room again.. eventually she will put you on a "Quest".
To solve it, go to Elena's house during the Day time, kitchen, speak to Natalie and then go to the play ground at night, rest should be self explanatory if reading the dialogue
Have fun
