I sent my sister-in-law a check for her birthday 3 months ago and she never cashed it. When will it be safe to add it back into my account?

Maybe ask her don’t hand out checks no Moore 

AFTER you put a stop payment on the check. 

The first thing you should do is to call her and ask if she got it.

You CAN'T 'add it back' because the funds NEVER left your account. Unless there is an expiration date on the face of the check, it technically never expires. That said, banks are not required to honor 'stale' checks. Most banks consider a check stale after 6 months.

Your check becomes stale dated after one year.

Call your sister in law and ask her if or when she is going to cash your check