This is going to be a bit longer, so please be patient with me.
I am from a different European country and have lived in the UK for many years.
I have a partner and a 13 month old and I'm currently 4 months pregnant with my second.
My sister still lives in my home country. My mother is wealthy (but not superrich) and my sister lives in one of my my mother's apartments for free.
My sister has a 5-year old son with her husband and a baby girl that is just over 4 months old with her boyfriend.
The reason for her "cheating" is because her husband treated her quite badly after my nephew was born. He refused to move into her apartment after the birth and insisted to continue to live in his apartment which is only 5 minutes walk away from the hospital he works in. He said that it would be too long of a commute in the morning from my sister's place. So he only ever spent the weekend at my sister's place and comes to visit on a Wednesday. On the day he is there, he doesn't really spend as much time as he should with his child. He uses the weekend to go cycling or use his motorbike. Nevertheless, my nephew is very attached to him.
He earns quite well, but he keeps my sister on a short "financial leash". I think he only gives her 300 Euros a month. If they divorced, he would have to pay her much more.
He also disrespects her and called her fat after giving birth.
My sister has a part time job, in which she only earns 900-1000 Euros a month.
But because she has low expenses and no mortgage or rent money, she is still able to save a few hundred Euros a month.
Child benefit is also very generous in my country.
She also eats for free everyday at my mother's place. My mother looked after my nephew when my sister was working for the first three years of his life.
When he started nursery part-time at the age of 3, my mother still looked after him in the afternoon so that my sister can nap after coming home from work. My mother also very often showers him in the evening.
My mother lives in the apartment above my sister.
When my nephew was 1, my sister started seeing her boyfriend. I can totally understand why she did it, he treated her badly and bsaiclly abandoned her after giving birth. Her husband went ballistic when he found out. He doesn't allow my nephew to spend anytime with her boyfriend, because he says her boyfriend is a pedophile (of course he isn't, he just says that because he is bitter).
My sister still refuses to divorce her husband. Her husband has a key to her flat and still hangs out there. Her boyfriend moved into a house close to my sister's place. He would like to move in with my sister and lead a normal family life. But my sister doesn't take the steps to let that happen.
So that's why she brings their daughter to his place every day so that he can see her.
My mother thinks that my sister is scared of her husband, as he threatened he will ruin her financially if she goes for a divorce.
I don't see my sister as a victim though. My sister once told me it's clever to be married to her husband because she will get his pension if he dies before her. For information, her husband's job is well paid, whereas her boyfriend works in a warehouse. She also eats breakfast with her husband on the weekend and although they fight a lot, I saw them sitting in quite a familiar way on her sofa on a Saturday night last time I visited, chatting like an old couple.
I feel sorry for her boyfriend, because he has lots of pictures of my sister at his place and I think he genuinely loves her. I always jokingly say to my mother that his whole place seems to be shrine for my sister.
I find my sister very selfish. I think neither her or her husband have a very good character. When my son was born, my mother came over to help me. It's only 1 1/2 hours by plane and my mother was still able to travel despite the pandemic at that time. My sister was massively pissed off about my mother coming over and helping me. My mother ended up staying 7 weeks because I lost over 2 litres of blood during birth and I was very weak. The longer my mother stayed, the angrier my sister got. She apparently said to my mother about me: "Why are you helping her, she's got nothing to do with our life". It's not like she didn't have childcare when my mother stayed with me. My father massively helped with my nephew in this time.
My mother also sometimes gives my nephew presents or buys him clothes or even pays for a holiday for him, as she thinks my sister can't afford it. When my son was born, my mother started writing down the amount of money she spends on my nephew. She says she has to give the same to my son, because otherwise it would be unfair. My sister doesn't like that either. She says: "You shouldn't give anything to her, she has a partner that earns well".
When my sister came over to visit 2 years ago over Christmas, my nephew was still toilet training. We were in the city centre and he had an accident and didn't wear a nappy, so he peed himself. My sister got angry because he didn't tell her he had to go to the toilet. She didn't have replacement trousers for him with her, so she forced him to walk around in wet trousers. I felt sorry for him, so I went into Primark to purchase new trousers for him. She never gave me the money for it. She said that I could buy new trousers for him, but she won't.
We also went to Thomas Land in Drayton Manor. We went to one of the children's casinos and we had lots of fun in there. In the end I ran out of cash so I asked my sister if she had a spare pound for me. My sister gave it to me, but she got angry in the face and said: "Can you get something for my son then? What will you buy for him?" I was really shocked, as I only asked for a pound and I had already spent hundreds of pounds on my nephew in presents since he was born.
When I criricize my sister's behaviour in front of my mother, she agrees to an extent, but then always sticks up for her. She says that I should "have more mercy with her". She says that my sister is this way because our father walked out on us as children. My mother and father made up a couple of years ago and now live together again as friends, not as a couple though.
She's 31 and I am 34. So I think it's a cheap excuse to use our childhood experiences as an explanation for her behaviour.
AIBU to think that my sister isn't a victim, but hasn't got a very good character and isn't a very nice person?