Yes, that's fine. Your wife might be advised to say at check-in at IBZ that you are not travelling/checking in.

However, if you plan on taking the same AMS-IBZ flight that your family is booked on, but travelling on a separate second IBZ-AMS-XXX ticket while you still also have a "valid" open return on your IBZ-AMS-BRS ticket, then this will cause problems. You're double-booked on the same flight - and that's not logical.

If the plan is to travel back from IBZ as far as AMS with your family, then perhaps the best thing to do is this: call whoever issued the ticket (KLM if bought on their website, or contact whichever travel agent made the booking if you used someone else - though, in this latter scenario, they are likely not going to be much help and/or may refuse to do this), have them "split" the PNR, so that your ticket is separated from that of your family; once that is done, you can change your ticket (upon payment of the change fee, as well as the fare difference) so that instead of flying IBZ-AMS-BRS on the way back, you will fly IBZ-AMS-XXX, while your family continues on the currently-booked AMS-BRS flight without you.

I would advise you to do this now. I don't think this is something you can do once travel has commenced.

Alternately, if you want to travel back with the family on your existing ticket only as far as AMS - and then make a self-connection there to a separate ticket to your new destination - make sure that you have no checked luggage checked in under your name. You will not be able to retrieve any checked bag on your name that was checked in at IBZ for BRS, and if you then don't board your AMS-BRS flight, they will have to delay the flight to remove your checked luggage item. In this scenario, too, it would be advisable if your wife informs the gate agent prior to boarding in AMS that you are not going to board the flight to BRS.

Please report back. People come here asking for help, but then never come back to share their experiences and to let us know how things worked out.