Is this sentences punctuation correct?

One employer or more?

One sister only?

One or more companies?

one employer with multiple sister companies

@Rememberme: You are not talking about the sister of the employers?

If so, you answered your own question.

Depends upon the context. If the sentence stands alone, you first of all need to specify what “this” is.
Since you have one employer with multiple sister companies, that is incorrect. It is your employer’s sister companies.
If you had many employers, 1 sister company, it would be employers’ sister company, and if you had 1 employer, 1 company, it would be employer’s sister company. Apostrophe is before s if singular, after if plural.

“__Is__ this __sentence’s__ punctuation correct?”
:)

If you mean you designed signs for the sister companies of the company that employed you, say it this way for clarity and simplicity:

“This temporary job also consisted of included designing signs for sister companies.”

This assumes that you have established context by identifying the company you were working for at the time.

Your question is not even correct. Sentence’s. Employer’s sister’s. Take Jeruba’s advice and just change the sentence.

@perspicacious: Apparently the OP is not talking about the employer’s sister but the employer’s sister companies.

I read it initially as the companies of the sister of the employer, or the company of the sisters of the employer/s. Under any circumstances, lots of ambiguity.

“This temporary job also consisted of designing signs for my employers’ sister companies”

This temporary job also consisted of designing signs for my employer’s sister companies.
I see no problem with it now. You may not have to use the word, “also” while instead adding , “in addition, this temporary job….” etc.