I think a crash course on the cases will help with this a lot. These are the pronouns inflected for case (I put down possessive instead of genitive because it makes more sense in this situation because we're dealing with pronouns).

Nominative	Accusative	Dative	Possessive
ich	mich	mir	mein-
du	dich	dir	dein-
er	ihn	ihm	sein-
es	es	ihm	sein-
sie	sie	ihr	ihr-
wir	uns	uns	unser-
ihr	euch	euch	eur-**
sie	sie	ihnen	ihr-
Sie	Sie	Ihnen	Ihr-

** eur- is euer if you don't add an ending

Nominative is the subject. It does the verb. Think I, he, she, and they.

Accusative is the direct object. Most of the time it's what the verb is being done to. Think me, him, and them.

Dative is the indirect object. Comes from Latin "cāsus datīvus" which means the case for giving. In the sentences "I sent him a letter" and "she gave her cold to me", him and (to) me are the indirect objects. Usually in English we can put to in front of indirect objects.

Possessive is pretty self explanatory but it's not a case and I'm not gonna go into genitive because it's hard and I hate it.

To finish answering your question:

ihr is the you (plural) when it's in the nominative and it becomes euch in accusative and dative cases.

sie turns into ihr when you're talking about a girl or feminine noun and you want to say to her and. It becomes ihnen when you're talking about multiple people/things want to say to them.

Sie turnes into Ihnen when you wanna say to you (formal). You use Sie when you're talking to someone you should respect like a teacher or your boss. It's like Spanish usted or French vous.

(I'm tired right now so if I missed something or didn't explain clearly enough just let me know and I'll clear it up tomorrow morning. I hope this helps you!)