Hello

We are putting in a new bathroom in the middle of our house.
We would like to put an in-line vent.
Toilet and sink will run in single pipe over to wall and then 90 to existing plubing.

Can we place in-line vent under sink or after both toilet and sink have joined in single pipe?

Not sure if that is enough information. Trying to keep it short.

Thanks

Can we place in-line vent under sink or after both toilet and sink have joined in single pipe? By "inline vent" I assume you mean a AAV, (see image) Yes you can, If local codes permit. If there's a tub or shower connect to the lavatory drain where it will be wet vented. Most bathroom groups are roughed in like this.
Toilet connects to sewer main or the stack vent. Lavatory connects to toilet drain and installs a AAV just downstream from the trap,(see image)oThe toilet wet vents through the lavatory vent and the tub/shower connects to the lavatory drain and is wet vented by it. This is a normal rough in and is acceptable both by local and state codes and also The Standard Plumbing Code Book in 90 percent of the country. Check your local codes to make sure you're not in the excluded 10 percent. Or that a AAV's permissible.
Good luck, Tom