I need help with some sentences

[Hyva]

Is that really onbvious though? (serious question).
To me it seems equally likely that ym grandmother would be doing something with my little sister. They have the same relation after all and them doing something together doesnt seems unlikely.
I wouldn’t know if it was the one or the other. But maybe practiced japanese readers would.

Also would おばあさんと彼女の妹 be possible? Im no good with japanese pronouns either.

[NathaLire]

This sounds like it’s too much. I would also vote for just おばあさんは妹さんと一緒に川で

さん added like @Kazzeon said, I think that sounds good

[Leebo]

I vote for no さん. I agree with @arisaclear that it would be assumed to be her younger sister unless otherwise specified.

[Merriad]

I’m sure a lot of this has been commented on above, but here are my edits, hope it helps.

In the morning, Tanjiro went from Kyoto to Tokyo by train.
朝に丹次郎さんは電車で京都から東京へ行きました。
Note: へ is a particle used to indicate the direction towards which some directional movement or action proceeds, for example「私は先週京都へ旅行に行きました。」(From A Dictionary of Basic Japanese Grammar, pg 116)

Granny was washing clothes in the river with her sister.
おばあさんは妹と川で服を洗っていた。
Notes: If you aren’t close with granny and want to convey respect to her younger sister (who may also be older than you) you can say 妹さん. It might seem redundant but people do it to extend respect. And 洗う is a bit better than 洗濯 since no washing machine is used, but it’s not a big deal either way.
*EDIT: using 妹さん instead of just 妹 would make it clear that you’re talking about granny washing clothes with her little sister, as opposed to yours. I agree that using おばあさん again sounds repetitive.

I was alone at my house. I was eating cold ramen.
私は家で一人でした。冷たいラーメンを食べていた。
Notes: A bit more natural to use でした (past tense of です) here indicating the state of being alone instead of いた. For example, when people ask “are you home alone now?” they say “今家で一人ですか？” Also, since you say in English “I was eating” instead of “I ate” you want to use the past progressive 食べていた. 寒い would also not be used here, for tactile objects like food, drink, objects, etc you use 冷たい, whereas 寒い is for the climate/someone feeling cold.