Mi portátil es más pesado que el de mi hermana.

Yes. In Spain, portátil is a very common way to say laptop. It is short for ordenador portátil.

• ordenador = computer
• portátil = portable

So mi portátil means my laptop. In Spain, this sounds very natural. In much of Latin America, learners may hear laptop, computadora portátil, or portátil, depending on the country.

Más ... que is the standard pattern for more ... than in Spanish.

So:

• más = more
• pesado = heavy
• que = than

Putting it together:

• más pesado que = heavier than / literally more heavy than

This same structure works with many adjectives:

• más grande que = bigger than
• más rápido que = faster than
• más caro que = more expensive than

Because pesado agrees with portátil, and portátil is masculine here.

In Spanish, adjectives usually match the noun they describe in gender and number.

• el portátil pesado = the heavy laptop
• la maleta pesada = the heavy suitcase

So in the sentence:

• Mi portátil es más pesado...

the adjective is masculine singular because portátil is masculine singular.

Here, el stands in for the noun portátil so that Spanish does not need to repeat it.

The full idea is:

• Mi portátil es más pesado que el portátil de mi hermana.

But Spanish often avoids repeating the noun when it is obvious, so it becomes:

• Mi portátil es más pesado que el de mi hermana.

So el de mi hermana means my sister’s one or more naturally my sister’s laptop.

Because de mi hermana by itself only means of my sister or belonging to my sister. It does not include the idea of the one.

Spanish needs the article el here to replace the omitted noun:

• el de mi hermana = the one belonging to my sister

Without el, the phrase would be incomplete in this sentence.

Here de shows possession, similar to of or belonging to in English.

So:

• el de mi hermana = the one of my sister
• more natural English: my sister’s one or my sister’s laptop

This is a very common Spanish pattern:

• el de Juan = Juan’s one / Juan’s
• la de María = María’s one / María’s

Because pesado here describes a characteristic of the laptop, not a temporary state or location.

Spanish usually uses:

• ser for identity, classification, and more permanent descriptions
• estar for location, condition, or temporary states

So:

• Mi portátil es más pesado... = My laptop is heavier...

That is a description of the object, so ser is the correct verb.

Yes, absolutely. That is also very natural.

There is a small difference in focus:

• es más pesado = is heavier
• pesa más = weighs more

In everyday English, those are often basically the same idea in this context. Both are correct and natural.

Because mi portátil and mi hermana are two different possessive relationships:

• mi portátil = my laptop
• mi hermana = my sister

Spanish needs both to make the meaning clear. If you removed the second mi, you would get:

• el de hermana ❌

which is not correct. You need:

• el de mi hermana = my sister’s

Yes. Portátil is an adjective ending in -l, so it does not change form for masculine vs feminine in the singular.

For example:

• un ordenador portátil
• una consola portátil

The word stays portátil in both cases. Only the article or the noun shows the gender.

In the plural, it changes normally:

• portátiles

It has a written accent because of Spanish stress rules.

• portátil is stressed on the second-to-last syllable: por-TÁ-til

Without the accent mark, Spanish pronunciation rules would place the stress differently. The accent shows exactly where the stress goes.

This is useful both for pronunciation and spelling.

Yes. The article must match the omitted noun.

For a masculine noun:

• Mi portátil es más pesado que el de mi hermana.

For a feminine noun:

• Mi tableta es más pesada que la de mi hermana.

So:

• el de... replaces a masculine noun
• la de... replaces a feminine noun

Yes, it sounds completely natural.

A speaker in Spain would understand it immediately, and it uses very normal grammar:

• Mi portátil = common in Spain
• más pesado que = normal comparison
• el de mi hermana = natural way to avoid repeating the noun

So this is a good, idiomatic sentence for everyday Spanish.