Future subjunctive in this case would actually require the initial verb to be in the future: esperaré que no estuviere enferma mi hermana, but that's not going to be commonly heard.1 There are several ways you can represent the future in a clause that must go in the subjunctive:
- Use the present subjunctive with or without temporal indications
As you already noted, this is probably the most common. If you feel there may be confusion between a future reference and a present one due to lack of context, you can just add in some adverb or adverbial clause indicating the correct temporal reference:
Espero que no esté enferma ahora (present reference)
Espero que no esté enferma la semana que viene (future reference)
- Use present subjunctive with periphrastic future
Use the verb ir along with the preposition a and your main verb in the infinitive. This construction creates a future aspect. It's not very common, though.
Espero que no vaya a estar enferma.
- Just use plain future
There are some grammarians who classify future and conditional in separate moods, and to me this is one of the good evidences as to why future shouldn't be considered strictly indicative:
Espero que no estará enferma.
In order of commonality, you'll find present subjunctive the most, followed by simple future, followed lastly by the periphrastic. And based on NGrams, the proportion of usage is about 75/23/2, but that doesn't take into account the present-tense senses of the present subjunctive, so the simple future may be a bit commoner.
1. Note even in Portuguese where the future subjunctive is alive and well, you can't say espero que não estiver doente, it would be either espero que não esteja doente with present subjunctive or espero que não vá estar doente with present subjunctive and periphrastic future