I am just a beginner but i think that 'Seine' is a male owner of a female or a plural thing. Sein would be a male owner of a male or neuter gendered thing. ihr is a female owner of a male or neuter noun. ihre is a female owner of a female or plural noun. Ihr with capital 'I' means the owner is 'you polite'. I am feeling it is a minefield too, but hoping by taking one step at a time and then maybe starting again from the beginning, I will eventually get the hang of it!
"Sein" is for one male owner = his; "ihr" is for one female owner = her. Then you have the endings for gender, number and case of the thing owned: "sein", "seinen", "seinem", "seine", "seiner" all translate to his; "ihr", "ihren", "ihrem", "ihre", "ihrer" all translate to her.