If you can book all seats in one transaction then you'll be seated together (dans la mesure du possible). Otherwise no such guarantee.

Which trains are you trying to book? Which date? We might be able to give more advice.

Within Germany you may use DB Navigator (or  bahn.com) which have a nice seat map.

We will likely take TGV (SNCF) and ICE (DB) trains. For the dates, it would be from August 24th (outbound journey in one day / no night train) to August 31st.

I saw that on the interail.eu website, it is possible to reserve seats for ICE and DB trains. Should I use this site to book my seats, or should I use the railway companies' sites (DB Navigator, etc.)?

Thanks you for your response

Interrail adds 2€ fee per person per train so it quickly adds up! Optional reservations (in Germany or elsewhere) are also more expensive.

Best to go through the companies when possible. Rail Europe works well for TGVs and cross-border services to Germany.

DB costs 4.90€ per journey and there's a seat map. ÖBB is 3€ per train.

Follow the guide here: https://www.seat61.com/interrail-and-eurail-reservations.htm

Note that part of the Rhine railway is closed for 3 weeks in August. High-speed trains from Paris are rerouted via Saarbrücken and skip Strasbourg. Book well in advance in any case.

As a side note: SNCF has an unofficial offer where they give free TGV reservations to Interrail travellers from France. Only applicable on domestic journeys and you must go to a counter (billet parcours d'approche).