Моя сестра говорит, что майка из хлопка удобнее, потому что такая ткань лёгкая и приятная.

Because сестра is a feminine singular noun, so the possessive adjective must agree with it.

• мой = my, masculine singular
• моя = my, feminine singular
• моё = my, neuter singular
• мои = my, plural

So:

• мой брат = my brother
• моя сестра = my sister

This is a very common structure in Russian:

• говорит = says / is saying
• что = that

So Моя сестра говорит, что... means My sister says that...

Russian often uses что after verbs of speaking, thinking, knowing, etc., just like English uses that:

• Он знает, что... = He knows that...
• Я думаю, что... = I think that...
• Она сказала, что... = She said that...

Not exactly. Майка usually means a tank top, undershirt, or sleeveless shirt.

It is not the most standard word for a regular T-shirt with sleeves. For that, Russian more often uses футболка.

So:

• майка = tank top / undershirt
• футболка = T-shirt

Depending on context, English translations can vary a little, but майка usually suggests something lighter and sleeveless.

Because the preposition из requires the genitive case.

• dictionary form: хлопок = cotton
• genitive singular: хлопка

So:

• из хлопка = made of cotton / from cotton
• из дерева = made of wood
• из металла = made of metal

This is a very common pattern in Russian for materials.

Удобнее is the comparative form of удобный:

• удобный = comfortable / convenient
• удобнее = more comfortable / more convenient

Russian often prefers this short comparative form instead of более + adjective.

So:

• эта майка удобнее = this shirt is more comfortable
• эта майка более удобная is possible, but less natural in many everyday contexts

Also, удобнее does not change for gender or number here. It works as a comparative predicate.

Russian can leave the comparison implicit if it is clear from context.

So майка из хлопка удобнее means:

• a cotton tank top is more comfortable
• or is more comfortable than others / than some alternative

If you want to state the comparison explicitly, you can add it:

• Майка из хлопка удобнее, чем синтетическая.
  = A cotton tank top is more comfortable than a synthetic one.

Without чем, the sentence still sounds natural.

Такая here means such a or this kind of.

It refers to the type of fabric just mentioned: cotton fabric.

So потому что такая ткань лёгкая и приятная means something like:

• because that kind of fabric is light and pleasant
• because fabric like that is light and pleasant

If you used эта ткань, it would point more directly to this specific fabric.
Такая ткань is slightly more general and descriptive.

Because they describe ткань, and ткань is a feminine noun.

In Russian, adjectives must agree with the noun in gender, number, and case.

• ткань = fabric, feminine singular
• therefore:
  • лёгкая = light
  • приятная = pleasant

Even though ткань ends in a soft sign ь, it is feminine. Many Russian feminine nouns end this way:

• дверь = door
• ночь = night
• вещь = thing

Here приятная means something like:

• pleasant
• nice
• comfortable to the touch
• agreeable

When talking about fabric, it often means the material feels good on the skin.

So in this sentence, лёгкая и приятная suggests cotton fabric is:

• light
• pleasant to wear or touch

Because both introduce subordinate clauses.

The sentence structure is:

• Моя сестра говорит, что...
  The clause after что is a content clause: what the sister says.
• ..., потому что...
  The clause after потому что gives the reason.

Russian punctuation regularly puts commas before these conjunctions:

• Я знаю, что он дома.
• Мы ушли, потому что было поздно.

So the commas here are standard and necessary.

The word is pronounced with stress on ё: лЁгкая.

A few useful points:

• ё is usually pronounced yo
• it almost always carries the stress
• in many printed texts, Russians sometimes write е instead of ё, but the correct form is still understood from context

So:

• лёгкий = light
• лёгкая = feminine form

For learners, keeping the ё helps a lot with pronunciation.

This word order is quite natural and neutral.

Russian word order is flexible, but changes usually affect emphasis, not the basic meaning.

Neutral version:

• Моя сестра говорит, что майка из хлопка удобнее, потому что такая ткань лёгкая и приятная.

Possible variations:

• Моя сестра говорит, что удобнее майка из хлопка...
  This gives extra emphasis to удобнее.
• Потому что такая ткань лёгкая и приятная, моя сестра говорит, что майка из хлопка удобнее.
  Grammatically possible, but less natural in normal conversation.

So the original sentence is a good standard model.

Yes. Удобный is a very common Russian adjective with a broader meaning than English comfortable.

Depending on context, it can mean:

• comfortable clothing or furniture
• convenient time, place, method, arrangement

Examples:

• удобная майка = a comfortable shirt
• удобное кресло = a comfortable armchair
• удобное время = a convenient time
• удобный способ = a convenient method

In this sentence, because we are talking about clothing and fabric, comfortable is the most natural meaning.