"My older sister lives in Mexico."
Translation:Mi hermana mayor vive en México.
The "personal a" is used before people who are direct objects. "My older sister" is the subject of this sentence.
Mi hermana es bonita. (subject)
Admiro a mi hermana. (direct object)
Another time you would see "a mi hermana" is if you have an indirect object pronoun to clarify:
A mi hermana le gusta leer. My sister likes to read.
Le doy mis libros a mi hermana. I'm giving my books to my sister.
In English, "my sister" is the subject in both "My sister likes to read" and "My sister lives in Mexico". No difference.
In Spanish, "mi hermana" is the subject in "Mi hermana vive en México", but the indirect object in "Leer le gusta a mi hermana", often reordered as "A mi hermana le gusta leer". Hopes this helps.
I've really struggled to get my head round this use of personal 'a' and I think it causes most native English speakers a lot of problems because we don't have anything like it in English.
I've personally found marcy65browns explanations to be really helpful in understanding it.
Here's quick and dirty way of remembering when to use it. If you find a person or people at the end of a sentence with for example, my, he, their, her, the in front it's going to need that personal 'a'.
A few examples:
You call my sister - Llama a mi hermana
They defeat their enemies - Ellos derrotan a sus enimigos
She calls the police - Ella llama a la policia
He treats his employees well - Él trata bien a sus empleados
He calls his wife - El llama a su esposa
You use "a" with "gusta" and "encanta" because gusta literally means "is pleasant to" and encanta "is enchanting to". The subject of gusta and encanta is not your sister or grandfather, it is whatever she or he likes or loves. "Mi hermana vive" is a direct translation of "my sister lives". Hermana is the subject of vive. "A mi abuelo le encanta mirar peliculas" could be translated literally by "watching movies is enchanting to my grandfather" (or, in Spanish word order, to my grandfather is enchanting watching movies". Mirar peliculas is the subject, and abuelo is an indirect object, "to him". I hope this is clear, I know it's confusing!
I tend to forget that most adjectives that end in a consonant do not change according to gender, so I sometimes hesitate with an adj such as mayor. I think it's because there are exceptions, such as trabajadora, in addition to that I need more practice. I agree that the hard x is irritating. It's the computer, so not much that can be done.