https://www.flyertalk.com/forum/luft...e-do-i-go.html

You are travelling in business class of Lufthansa, I presume.

None of you have Star Alliance status.

You will have access to Welcome lounge by arriving on an intercontinental business class. Take care that this is landside. You have to exit the airport and then enter again.

You also have access to any of Business Class lounges and the Panorama lounge. You will want to choose a lounge near your gate for Oslo, this will be one of Business Class Lounge near A13, Business Class Lounge near A26, Panorama Lounge.

Senator lounges are only for Star Alliance Gold status holders or American Express Platinum card holders travelling in Business class. If you and your wife have Amex Platinum cards, you can avail the Senator lounge near A13, but I would still suggest to go to one of the lounges near A26 if closer to your departure gate.
Business class passengers have no guesting privileges.

If your sister-in-law holds an Amex Platinum, she can enter a Business class lounge with the onwards BP. That way you can all visit together. Otherwise you can attempt to purchase entry at the door, but you are not guaranteed success.