Is what became apparent to the questioner regarding matters of envy from her husband's sister a result of the Istikhara prayer, and should she leave the apartment?

If you have performed Istikhara, then be content with what transpires afterwards and ask Allah to make it good. If He diverts you from it, then supplicate to Allah to grant you something better. If He decrees it for you, then supplicate to Allah for blessings, and fortify yourself with the prescribed remembrances (adhkar) and supplications (du'a). Maintain a good opinion of Allah Almighty and trust that He will protect you if you consistently perform the fortifications and remembrances.