My father passed away and since then my sister’s mother’s behaviour has been erratic

Question: I am in my 30s and have a half-sister (seven years old now) from my fathers second marriage. She was born when I was living abroad, and I have only seen her on a few occasions. Sadly, my father passed away a few years ago and since then her mothers behaviour has been very erratic. My aunt, who lives close by, constantly helps with my sisters education but berates my sisters mother. The mom indeed has learning difficulties and is far from an attentive mother and my aunt keeps pressuring me to intervene and take my sister to live abroad with me.
I feel very bad about this situation. I dont want children and I have no interest in adopting my sister when she has a potentially capable mother and a fat pension my father left for them both. I am now avoiding my aunts calls and I resent her for insisting I do something.