Does cash top-ups to one another's CPF accounts for loved ones also qualify for a tax rebate?

Yes that is correct. Both siblings will have $14K in SA and $14K tax relief in this case.(not tax rebate)

Edit: To be clear, please note that the tax relief on siblings will not apply if the siblings have an annual income of more than $4K, unless they are handicapped.

The allowable parties with respect to topping up for tax relief are: your parents, parents-in-law, grandparents, grandparents-in-law, spouse and siblings. But you can actually top up for anyone with cash.

Take note that you can ONLY earn tax relief on cash top-up to your Sibiling ... IF

His or her income in the preceding year of top-up is $4,000 or below. (This includes salary and even tax-exempt source such as bank interest, stock dividends, pension etc)

or
He or she is handicapped.

So in your mentioned case, unless both parties are not working, they would only earn $7k tax relief for the portion on self top-up.