If you have line of site to your sisters house, there are relatively inexpensive technical solution. However to do it leagally, there maybe signifiant municipal regulations that need to be mitigated. Also, most residential broad band contract prohibit sharing of service in that way. You may need to create your own isp. Not impossible but maybe not worth the headache.
Do your parents have cell phone coverage? If so, that might be simpler.
Do your parents have cell phone coverage? If so, that might be simpler.