After arriveding she had called me to say our mother is sick and very weak.

I'm sorry to hear this.

My sister helped our nephew toby explain hering maths he sometimes finds hard to understand.

His mother doesn't care about her son.

I think is notdon't think she is a good mother. She almost she abandont the houseed her son and ther son. house. There are those who have happiness and do not appreciate it... What a difficult time we live in!.

After she arrived she had called me to say our mother is sick and very weak.

My sister helped our nephew to explainteach her maths that she sometimes finds hard to understand.

His mother doesn't care about her sonim.

I amfeel so sorry aboutfor my sister-in law.

I think she is not a good mother. She almost she abandonted the house and her son. There are those who have happiness and do not appreciate it... What a difficult time we live in!.

Today my sister traveled to Algiers to see our family.

Correct, but "went to Algiers" or "flew to Algiers" is a little more natural. I would use travel if I was talking about the whole trip, instead of just going from point A to B.

After she arrived, she had called me to say our mother is sick and very weak.

Or... "After she arrived, she called to tell me that our mother is sick and very weak."

My sister helped our nephew to explain her mathsexplain maths to our nephew, which he sometimes finds hard to understand.

His mother doesn't care about her son.

I am so sorry about my sister-in -law.

I would use this sentence if I was apologizing to somebody for my sister-in-law. Are you trying to say "I feel sorry for my nephew."?

I think she is not a good mother; she almost she abandonted the house and her son. There are those who have happiness and do not appreciate it... What a difficult time we live in!.

But the problem is within ourselves...