Val your sister is impossibly selfish and she has to go. i do not want to tell you what to do but having your sister there is just causing you to have a poisoness environment which in no way will help get pregnant. the stress from that itself is too much for you. if she can afford to have a second baby, she can afford to get out and live on her own. a condom is cheaper than diapers! i am all for having babies but your sister is making your life difficult and making you look after her family. there really is just too much stress in your life. you have to tell her to leave. this will eventually break your family if you actually keeep them under your roof. her comments were really not nice and as a sister to her sister, she doesnt deserve all you are doing for her. she has to go.
your stress levels affect how your body works and with the way your sister is, i dont see you ever being healthy to just start trying nicely.
im so sorry for you