There is a very long backstory to this and I don't want to drip feed so I will try to keep it brief.
My mum suffered a chronic illness and went into residential care in her late 50s. My dad started a relationship with another woman about 18 months later. He did not tell me or my siblings for about 2 years and we did not meet GF.
GF was told by him that he was separated from DM. This was not true. he attended my sister's wedding with my DM only 2 weeks before her death.
DM died 2 years ago and since then I have asked to meet GF on several occasions ( would not have felt comfortable meeting GF while DM was still alive as would have had to lie about too much stuff). DF has continually stalled and come up with reasons why we cannot meet.
Today he called to ask what DGC would like for Xmas. I asked DF what he would like and he told me and then added "and GF would like X book". I feel a bit 
about buying a present for someone I have never met although I know she helps DF buy presents for DGC but then he said " and she says it doesn't matter if the book comes from a charity shop as it is quite old and may be out of print but she would prefer it to be an animal charity."
For some reason I am really really irritated by this. I don't support animal charities as a general rule. I support charities for children, the aged , research into the illness which killed my DM and mental health but not animals. I am annoyed at being told which charities should get my money. To be honest if I bought the book I would buy it new as it only costs about £6 anyway.
But AIBU to be annoyed:
a) at being asked to buy a present for someone I have never met ( I have written to her and asked to meet - no response);
b) at being told which charities to support
FTR last year I bought DF a gift voucher for a day out for 2 people so it was sort of like buying her a present although I later found out from DF that she didn't go because it was a wine-tasting event and she doesn't drink.