Your mother is clever, isn't she? To move to another country when elderly is quite an undertaking, and I would say your mother has a will of iron and is as tough as old boots.
Based on what you write it sounds as though she was not getting the attention she demanded from your brothers, who seized the opportunity to offload the responsibility on to you, because that is what sisters are for, isn't it?
You need to remind yourself frequently that your mother does not need your attention, although she may want it. She is safe, cared for and with plenty to occupy her; you do not have to worry about her eating habits, being ill, being vulnerable, being lonely.
She however, is determined to have you at her beck and call and she won't give up, so you have to be strong and set strict limits on how much time you give her. You have to resist her guilt manipulation by focusing on your family and job, and it will not be easy; I saw my domineering paternal grandmother ruin my parents' marriage, a friend's mother who died aged 104 thwarting her daughter's plans to start a new life following retirement, and a colleague who had to give up her job because her mother pestered her at work all day, everyday. All women whose only concern was themselves.
Start by refusing the daily phone calls, too busy with work.
I have no doubt after knowingly spoilt your day with her negativity your mother goes on her way, refreshed.