So did she get pregnant on her own? Was it an immaculate conception? If there's reason to get annoyed with anyone, it should be her husband, who should've made sure she shouldn't have got pg so quickly so soon after the Doctors advised her not to. For all you know, she wanted to wait but he pressurised her into it.
TBH, you sound jealous that your husband gave her a lift - but she is not just some other woman, she is her sister, his flesh and blood, who has had a horrendous time of it, medically and emotionally. If my DP didn't want to offer help to his siblings when they were in need, I'd be disgusted with him, not resentful that he is caring. To say that you'd not expect a lift of her husband is irrelevant - he is not your brother; your DH is hers. As for him being fed up of hearing about it, all siblings get fed up with their brothers and sisters; it's no reason to feel so nasty about her. I think a grown man can cope with his sister being a bit annoying, don't you?
'Milking her illnesses' is just an awful thing to say. I would hate to be in her shoes and frankly, I would feel like I deserved a bit of a moan. Yes, she might have become a bit self-obsessed about it all, but I think most people would become a bit self-preoccupied and worrisome if that many health problems had happened to them in a relatively short period of time! Have you ever stopped to consider that so many serious things have happened to her that she's developed anxiety problems (or even Post-Traumatic Stress Disorder - it doesn't have to be massive things that trigger it). This would account for her talking about it a lot... although we only have your word that she is doing. IMHO, you sound like you're outraged that she has the cheek to get ill, like it's a personal offence to you. Are you sure you're not also jealous that she is getting more attention than you?
Instead of begrudging her a bit of consideration from her own brother and acting like she's enjoying the attention, you ought to thank your lucky stars you haven't had so many serious health concerns in your life. She could have died as a result of all her pgs and as a mother, she must be doubly terrified about the results of the lumpectomy.
Are you sure you and your family haven't just decided that she is 'trouble' or a problem-causer, because of her health? Poor woman.