ungrammatical, unidiom,and missing.

[Alanmo]

By the time he told his mom how badly his older brother had behaved, he almost forgot to tell her that there was a dog passed on the other night.

By the time he told his mom how badly his younger behaved, he almost forgot to tell her that her brother died in combat the other night.

Is there any problems with sequence of the tense ?

Thank you!

[Tdol]

I'd use 'had told'. Also, 'there was a dog passed on' doesn't work- try 'a dog had passed on'.

[tedtmc]

By the time he told his mom how badly his older brother had behaved, he almost forgot to tell her that there was a dog passed on the other night.

I'd use 'had told'. Also, 'there was a dog passed on' doesn't work- try 'a dog had passed on'.

Shouldn't the verbs in bold be used in the past tense?
My understanding is that the past tense is time-specific as in this case whereas the past perfect tense as you suggested is not time-specific.

[Tdol]

There a sequence here that I think that, together with the 'by the time' makes the past perfect run better. Hi didn't gorget- he almost forgot, so he told here after telling her about the brother's behaviour, which means that using the past perfect for the verbs in the beginning works better.