Su hermana también me cae bien; es trabajadora y bastante modesta.

Caer bien is a very common Spanish expression meaning to like someone / to find someone likable / to get a good impression of someone.

So Su hermana me cae bien means that the speaker likes the sister as a person.

A useful contrast:

• Me gusta su hermana = I like his/her/your sister
  • This can be neutral, but it can also sound more like attraction depending on context.
• Su hermana me cae bien = I like his/her/your sister; she seems nice to me
  • This is clearly about personality and your impression of her.

Because the structure is:

• alguien le cae bien a alguien

Literally, it works something like someone falls well to someone, but that literal meaning is not how you should understand it. The important thing is:

• me = to me
• te = to you
• le = to him/her/you
• nos = to us
• os = to you all (common in Spain)
• les = to them/you all

So:

• Me cae bien = I like him/her
• Te cae bien = You like him/her
• Nos cae bien = We like him/her

In the sentence, Su hermana is the person who is liked, and me shows who likes her.

Both can be translated as to like, but they are not always used in the same way.

Caer bien is especially common for people and means:

• you find them pleasant
• they make a good impression on you
• you get along with them / like them as a person

Gustar is broader and can be used for people, things, activities, ideas, etc.

With people:

• Me cae bien often sounds more natural when you mean I think she’s nice
• Me gusta can sometimes sound stronger, more personal, or even romantic depending on context

So in this sentence, me cae bien is the most natural choice for I like her as a person.

Su can mean all of these, depending on context:

• his
• her
• your (formal singular)
• their
• sometimes your (formal plural)

So su hermana could mean:

• his sister
• her sister
• your sister
• their sister

Context tells you which one is meant.

A useful Spain-specific note:

• If you are speaking informally to one person, Spanish from Spain would usually prefer tu hermana for your sister
• If you are speaking informally to several people in Spain, you would usually say vuestra hermana

So su hermana often sounds like his/her/their sister or your sister in a formal situation.

También means also / too, and its position is fairly flexible, but some placements sound more natural than others.

In this sentence:

• Su hermana también me cae bien

the word también naturally adds the idea of also to the whole statement: the sister is also someone the speaker likes.

This is a very normal position.

Other positions are possible, but they can change the emphasis:

• También su hermana me cae bien = more marked; it puts stronger focus on her sister too
• Su hermana me cae bien también = possible, but often sounds a little less neutral in this type of sentence

So the original word order is a very natural, standard choice.

Because they describe hermana, which is a feminine singular noun.

In Spanish, adjectives usually agree with the noun they describe in:

• gender: masculine or feminine
• number: singular or plural

So:

• hermano trabajador y modesto
• hermana trabajadora y modesta

Here:

• trabajadora agrees with hermana
• modesta also agrees with hermana

Here it means hard-working.

That is because in this sentence it is being used as an adjective after ser:

• es trabajadora y bastante modesta

So the meaning is:

• she is hard-working and quite/fairly modest

Compare:

• Es trabajadora = She is hard-working
• Es una trabajadora = She is a worker

So the presence or absence of the article can change the meaning a lot.

Because they are predicate adjectives after ser.

In Spanish, when you describe someone with adjectives after ser, you normally do not use an article:

• Es inteligente
• Es simpática
• Es trabajadora
• Es modesta

If you add an article, you often turn the word into a noun phrase instead:

• Es una trabajadora = She is a worker
• Es una modesta... would need a noun after it, because modesta is mainly an adjective

So in the sentence, es trabajadora y bastante modesta is simply the normal way to describe her qualities.

Here bastante means quite, fairly, or rather.

So:

• bastante modesta = quite/fairly modest

In this sentence, bastante is an adverb modifying the adjective modesta, so it does not change form.

Compare:

• Es bastante modesta = She is quite modest
• Tiene bastantes amigos = She has quite a lot of friends

In the second example, bastantes is an adjective meaning quite a lot of / enough, so it agrees in number.

Because trabajadora and modesta are being presented as personal qualities or characteristics, not temporary states.

• ser is used for characteristics, identity, and more lasting descriptions
• estar is used for states, conditions, and locations

So:

• Es trabajadora y modesta = these are qualities she has as a person

Using estar here would sound wrong in normal Spanish, because trabajadora and modesta are not usually temporary conditions in this context.

The semicolon links two closely related parts:

• Su hermana también me cae bien
• es trabajadora y bastante modesta

Each part could stand on its own as a sentence, so a semicolon is a neat way to connect them without fully separating them with a period.

It works a lot like in English:

• Your sister is nice too; she’s hard-working and quite modest.

A comma would usually be less formal and, in careful writing, not the best choice here because it would join two full clauses too loosely.

Not exactly.

Modesta usually means:

• modest
• not boastful
• unassuming

Humilde can mean:

• humble
• modest
• unpretentious

But humilde can also sometimes suggest:

• a lower social or economic background
• simplicity
• a humble position

So in this sentence, modesta is a good choice if the idea is that she does not show off or think too highly of herself.