Not really, stress in Russian is usually hard to predict and you have to learn its position when learning a word. Also it's mobile sometimes. So, for example, for the word сестрА (sister) the plural form is сЁстры. There the stress changes its position and causes some morphological changes as well (е > ё).
Not so strictly...say еле-еле is rather yeli-yeli,where unstressed second е sounds like i in kilt or milk. In перемена (change,break) there is no taste of ye at all and first е is like e in pet,while second one is i in palindrome. Third е again like in pet . The stress in the whole word is on the third syllable. First two е are both unstressed but are pronounced differently ( if you hear variant with "pet-pet-pet",not pet-milk-pet ,it means you met a dialect. Same with О. It's pronounced pA-gO-dA with stressed O. If you hear pO-gO-dA,be sure- you are in Vologda city or somewhere near Volga river. HTH))
It's because сесрта is feminine so you have to use "moya" (моя) as that's also feminine. To know which one to use you first have to know the gender of the noun and then you'll know whether to use мой (masculine), моя (feminine) or моё (neuter).
If the noun is plural then you use мои.
Hope that helps.