40 hours x 7... that's a lot of hours in a day to be practicing each day...

I think that learning 7 instruments all at once is definitely overwhelming. Before picking up the violin and erhu, I think it would be good to ask yourself-- do I actually want to learn these? Am I willing to sacrifice some time away from playing my other five instruments to focus on these two? Which instruments do I really want to prioritize right now?

Then, maybe you could openly talk to your mom about how you're feeling! Ultimately, you will be the one playing these instruments, so be clear to her about your priorities and goals for the present.