1 Peter 5:9 (Parsing and Commentary)

Greek (WH): ᾧ ἀντίστητε στερεοὶ τῇ πίστει, εἰδότες τὰ αὐτὰ τῶν παθημάτων τῇ ἐν τῷ κόσμῳ ὑμῶν ἀδελφότητι ἐπιτελεῖσθαι.

NEB: "Stand up to him, firm in faith, and remember that your brother Christians are going through the same kinds of suffering while they are in the world."

ESV: "Resist him, firm in your faith, knowing that the same kinds of suffering are being experienced by your brotherhood throughout the world."

ᾧ functions as the dative direct object of ἀντίστητε (Mark Dubis, 1 Peter: A Handbook on the Greek Text, page 169): this verb takes the dative as its object. ἀντίστητε is also aorist active imperative second person plural of ἀνθίστημι. Compare James 4:7.

Paul J. Achtemeier suggests that it's hard to determine whether στερεοὶ modifies the subject of the verb (i.e., "resist him, [you who are] firm in the faith") or bears an instrumental sense (i.e., "resist him [by being] firm in the faith"). See 1 Peter in the Hermeneia Series, page 342.

From the EGGNT for 1 Peter: "The dat. τῇ πίστει can be cstr. either as a dat. of respect (Elliott 860; Dubis 169) or a locat. of sphere, with little difference in sense. The clear implication of this statement, as elsewhere in the NT (cf. Eph 6:10-17), is that faith can triumph over evil." See Greg W. Forbes; Robert W. Yarbrough; Andreas J. Kostenberger. 1 Peter (Exegetical Guide to the Greek New Testament) (Kindle Locations 5317-5319). B&H Academic. Kindle Edition.

εἰδότες is a perfect active participle nominative plural masculine of οἶδα. Dubis argues that the participle here is causal and means "knowing that." See Luke 4:41; 1 Peter 1:18.

τὰ αὐτὰ τῶν παθημάτων-Selwyn thinks this genitival construction coupled with plural neuter words is unusual: he avers that it connotes "the same kinds of sufferings" (The First Epistle of St. Peter, page 238). 

τῇ ἐν τῷ κόσμῳ ὑμῶν ἀδελφότητι-The book, Going Deeper with New Testament Greek observes: (“ by your brothers in the world”) – This is another example of a “sandwich” construction. The article τῇ is grammatically connected with the noun ἀδελφότητι (“ brothers”). This noun is then further modified by the “stuff” in the middle (ἐν κόσμῳ ὑμῶν) explaining what type of brothers (i.e., the in-the-world-of-you brothers). τῇ ἀδελφότητι could be interpreted as a dative of respect (“ with respect to your brothers” or “in your brothers”), a dative of agent (“ by your brothers”), or a dative of disadvantage (“ on/ against your brothers”).790 Notice that the noun τῇ ἀδελφότητι (“ brothers” or “brotherhood”) is feminine and not masculine, which is common with abstract nouns.

See Köstenberger, Andreas J.; Merkle, Benjamin L; Plummer, Robert L. Going Deeper with New Testament Greek: An Intermediate Study of the Grammar and Syntax of the New Testament (Kindle Locations 9344-9350). B&H Publishing Group. Kindle Edition. 

ἐπιτελεῖσθαι-Mark Dubis offers these remarks on the morphology of the verb and its relationship to other words in the text (1 Peter: A Handbook on the Greek Text): 

Pres pass inf ἐπιτελέω (indirect discourse after εἰδότες, a verb of perception; see also 2:11 on ὡς παροίκους καὶ παρεπιδήμους ἀπέχεσθαι τῶν σαρκικῶν ἐπιθυμιῶν αἵτινες στρατεύονται κατὰ τῆς ψυχῆς). Translations such as “undergoing” (NIV, NRSV), “being experienced” (ESV), “going through” (TEV), or “enduring” (NET) fail to communicate the goal-orientation of ἐπιτελέω. This verb indicates that it is the completion of these sufferings that is in view. It is better, then, to understand this verse to affirm that these sufferings are being “accomplished” (see KJV, ASV, NASB) or “brought to completion” by Christians worldwide, which explains the reference in the next verse to suffering “briefly” (ὀλίγον). This verb appears in a final position in the infinitival construction for emphasis (on this word order, see 2:8 on ἀπειθοῦντες).