Aunque mi amiga es modesta y simpática, todas sabemos que canta mejor que nosotras.

Because aunque can take either the indicative or the subjunctive, depending on how the speaker sees the information.

• Aunque + indicative: the speaker presents it as a real, known fact.
• Aunque + subjunctive: the speaker presents it as uncertain, hypothetical, or not important to the main point.

Here, mi amiga es modesta y simpática is treated as true and known, so es is natural.

• Aunque mi amiga es modesta... = even though my friend is modest...
• Aunque mi amiga sea modesta... = even if my friend is modest... / although she may be modest...

So in this sentence, es fits the intended meaning best.

Because adjectives in Spanish usually agree with the noun they describe in gender and number.

Here the noun is mi amiga, which is:

• singular
• feminine

So the adjectives must also be singular feminine:

• modesto → modesta
• simpático → simpática

If the noun were masculine, you would get:

• mi amigo es modesto y simpático

In modern standard Spanish, a possessive like mi, tu, su, nuestro, etc. usually goes directly before the noun, without an article.

So you say:

• mi amiga
• tu casa
• nuestro profesor

Not:

• la mi amiga

Spanish does have a different possessive pattern after the noun:

• una amiga mía = a friend of mine
• la amiga mía = my friend (more emphatic or marked in tone)

But in a neutral sentence, mi amiga is the normal form.

Todas means all of us here, and it is feminine plural.

This tells you that the speaker is part of a group of females:

• todas sabemos = all of us women know

Also, sabemos is first person plural, so the speaker includes herself in the group.

Compare:

• todas sabemos = all of us women know
• todos sabemos = all of us know, with a masculine or mixed group

So todas is chosen because the group being referred to is female.

Yes. Spanish often allows words like todos, todas, muchos, algunas, etc. to stand on their own when the noun is understood from context.

So:

• todas sabemos

literally means something like:

• all of us women know
• we all know

The noun is omitted because it is not needed. Spanish does this very naturally.

Because Spanish uses saber to mean to know a fact or to know that...

• sabemos que canta mejor = we know that she sings better

By contrast, conocer means to know / be familiar with a person, place, or thing:

• conozco a tu amiga = I know your friend
• conocemos Madrid = we know Madrid

So:

• saber + que + clause is correct
• conocer + que + clause is not

Because Spanish often leaves subject pronouns out when they are already clear from the verb form or the context. This is called a pro-drop language feature.

So Spanish naturally says:

• sabemos rather than nosotras sabemos
• canta rather than ella canta

The pronouns can be added for emphasis, contrast, or clarity:

• Nosotras sabemos que ella canta mejor que nosotras

But without them, the sentence is more natural and less heavy.

Because mejor is the comparative form used with verbs to mean better.

• canta mejor = she sings better

Think of it as related to bien:

• canta bien = she sings well
• canta mejor = she sings better

By contrast, más bien usually means something like:

• rather
• more accurately
• fairly / pretty much, depending on context

So canta más bien would not mean sings better.

After a comparison, Spanish often uses que plus a pronoun:

• mejor que nosotras = better than us / better than we do

Here, nosotras is feminine plural because the comparison is with a group of females.

What is really understood is something like:

• canta mejor que nosotras cantamos

But Spanish often leaves the repeated verb out because it is obvious from context.

So:

• canta mejor que nosotras is a very normal way to say:
• she sings better than we do

Yes, you could. That version is grammatically correct.

• canta mejor que nosotras cantamos

But Spanish often omits the repeated verb when it is obvious:

• canta mejor que nosotras

The shorter version is more natural in everyday speech.

Including the second verb can sound:

• more explicit
• more emphatic
• sometimes a bit heavier stylistically

So both are possible, but the sentence you were given is the more idiomatic one.

Because the sentence begins with a subordinate clause introduced by aunque, and it is very common to separate that opening clause from the main clause with a comma.

So the structure is:

• Aunque mi amiga es modesta y simpática, = subordinate/concessive clause
• todas sabemos que canta mejor que nosotras. = main clause

This comma helps the reader hear the pause and see the sentence structure more clearly.

In short, it is standard punctuation for this kind of sentence.