haha ha kono surippa ha omotta hodo omoku nai si, sono kussyon mo suwariyasui to iimasita.

They belong to two different layers of the sentence.

• 母は = As for my mother, ...
• このスリッパは = inside what she said, as for these slippers, ...

So the structure is basically:

• 母は
  [このスリッパは思ったほど重くないし、そのクッションも座りやすい]
  と言いました。

In other words:

• outer は = topic of the reporting sentence
• inner は = topic inside the quoted statement

This is very normal in Japanese when one sentence contains another sentence as a quote.

Usually, yes.

When Japanese speakers talk to other people about their own family members, they often use plain family terms like:

• 母 = my mother
• 父 = my father
• 兄 = my older brother

So 母は ... と言いました most naturally means My mother said ...

This is a very common pattern:

• 思ったほど〜ない = not as ~ as (someone) thought
• more generally, X ほど Y ない = not as Y as X

So here:

• 思ったほど重くない = not as heavy as [I/she] thought

A literal breakdown is:

• 思った = thought
• ほど = to the extent / as much as
• 重くない = is not heavy

Together: it isn't heavy to the extent I thought
Natural English: it's not as heavy as I thought

The person who did the thinking is omitted. In context, it is often understood as the person speaking the quote.

Because い-adjectives make negatives in a special way.

• 重い = heavy
• remove the final い
• add くない

So:

• 重い → 重くない

This is the normal negative form of an い-adjective.

Other examples:

• 高い → 高くない = not expensive/high
• 寒い → 寒くない = not cold

し is often used to give one reason or to list multiple points.

Here it feels like:

• ... and ...
• ... and besides ...
• ... plus ...

So:

• このスリッパは思ったほど重くないし、そのクッションも座りやすい = These slippers aren't as heavy as I thought, and that cushion is easy to sit on too

A useful nuance: し often sounds like the speaker is giving one point and then adding another supporting point.

も means also or too.

• そのクッションも = that cushion too / that cushion also

It suggests that the cushion is being added to the discussion along with something else already mentioned.

So the sentence is doing this:

• point 1: these slippers are not as heavy as expected
• point 2: that cushion is also easy to sit on

Also note that も often replaces は or が.

So instead of:

• そのクッションは

you get:

• そのクッションも

because the idea is that cushion too.

It comes from:

• 座る = to sit
• 〜やすい = easy to do

So:

• 座りやすい = easy to sit on / easy to sit in

In natural English, depending on context, it may be translated as:

• comfortable to sit on
• easy to sit on
• pleasant to sit on

But literally it is about ease of sitting, not just general comfort.

と is the particle that marks quoted content with verbs like 言う.

• A と言いました = said A

So everything before と is the content of what was said:

• このスリッパは思ったほど重くないし、そのクッションも座りやすい
  • と言いました

Without と, the sentence would not clearly mark the quoted statement.

Because Japanese commonly uses plain forms inside quoted content, especially in reported speech.

Here, inside the quote, we have:

• 重くない
• 座りやすい

These are plain forms.

Then the main sentence ends politely:

• 言いました

This mix is very common:

• quoted content = plain
• main sentence = polite

You can quote polite speech too, but plain forms are extremely normal in this structure.

It is closer to an indirect quote in normal English style, though Japanese often uses the same と structure for both direct and indirect reporting.

Japanese does not always need quotation marks to show speech content.
So:

• A と言いました

can mean either:

• She said, “A.”
• She said that A.

In this sentence, English usually translates it more naturally as said that ...

Japanese nouns usually do not show singular vs. plural by themselves.

So スリッパ can mean:

• slipper
• slippers

Context decides.

In real-life translation, this slipper or these slippers may both be possible depending on the situation. Since slippers are often thought of as a pair in English, these slippers may sound more natural, but the Japanese form itself does not force a plural.

This shows the normal Japanese ko-so-a-do system:

• この = this ... near the speaker
• その = that ... near the listener / already mentioned
• あの = that ... over there, away from both

So:

• このスリッパ = these/this slippers/slipper here
• そのクッション = that cushion

The exact feeling depends on the situation, but the contrast tells you the speaker is treating the two objects differently in terms of perspective or distance.