I wrote two sentences each using " Ssince" and " For"for," respectively.

I hope that I am understood.

or

I hope that I am making sense.

"To make sense" is an idiom which is used often in the US.

My sister has been working inat the company since 1990 whoand my father also had worked there.

Today is the 79th day of my daily English challenge.

For a bit more advanced phrasing, try saying: “Today marks the 79th day of my daily English challenge. Personally, I would also capitalize the challenge name to “Daily English Challenge” to really express the idea that it’s a part of my daily routine (not necessary at all).

I hope that I can be understood. OR I hope that this is all understandable.

My sister has been working inat the company since 1990, whoere my father had also worked.

I am not sure if you mean that your father worked at the same company. That’s how I interpreted this sentence.

I hope that I understood and used them correctly.

It sounds odd to leave "understood" at the end of the sentence.

My sister has been working in the company since 1990 who my father had worked..

I think you might be trying to say something like "My sister has been working in the same company as my father since 1990" Or "My sister has been working in the company since 1990. My father also worked there." Not sure if you meant either of these but the first part of the sentence is correct