No, this is how it works. Tier is based on your own tier mileage earning. What other people in your family account earn makes no difference to it.

I couldn't find anything in the T&C re the tier miles they earn, but that should be the full amount for them. The actual miles they earn under the family bonus program is 20%, which is credited to your account.

The solution to this is to ask Skywards to remove them from your family bonus account (3 deletions allowed per year). This way they can start earning their own miles.

If you still want to go over the 25,000 mark to earn silver, you will need another flight, or try upgrading to F on one or more of the sectors and see if that helps.