it's so hard when nephews and nieces are involved. and I'm sorry you are going thru this. family can really suck at times.
mine is kinda the opposite though - I'm invisible almost all the time!

I would love to spend more time with my two nephews or my baby niece, but my two sisters don't include me in things. as if it takes too long to send an email or a text or do a phone call?!
and my younger sister just got engaged in November, with a wedding date set for july. and I haven't been told jack from her!!! the only way I knew the date of the wedding was my mom had written it down on a piece of paper for my Grandma (who is 95) and I happen to see it when I was there visiting her. and I didn't even know where it was gonna be until I got the invite in the mail a month ago! otherwise nothing. she can't even keep her own sister included on the details of the biggest day of her life?! anyways...
maybe try turning your phone off for a phone, so you don't get the texts or messages. just take a "me-day"! don't let the distractions your sister uses on you invade your day. tell yourself you are doing it FOR your nephews, cuz you don't want them to see you explode. and if you get worse, it doesn't help anyone in the end.
just remember, you have to take care of yourself - one way or another...