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There'S a handful of antology shows out there:
The Outer Limits, Amazing Stories, Are You Afraid Of The Dark, Goosebump, I'm affraid that's the best I can suggest.
I've checked the episode listing for all of these shows and followed up on anything that seems close, sadly none of them are the one I remember. Though Goosebumps "Strained Peas" had me hopeful, the sister however wasn't a baby and I think the Sister was in fact a year or two older than the boy.
Could it have been an episode of Eerie, Indiana or Freaky (New Zealand series aired in the 00s)? Was this from a show or film that was aimed more towards adults or kids? I was also considering an episode from Tales from the Dark Side or Monsters.
Could it be Child of Rage
? Its Wikipedia page mentions the grandfather incident.
For the life of me I can't remember this show/episode, but I'm 99% certain it was on a tv show like Tales from the Crypt, but I've checked their episode listing and it isn't on there.
1st the siblings are children. The girl no more than 12 and the boy like 10 maybe. It's in a everyday setting, no sci-fi or anything. I think the girl is possessed or something early in the episode (my memory is hazy here), or she's injured and her head trauma has her set on killing her brother. The boy does try to tell his parents but they disregard it for a long while (one of them does start to suspect near the end, father .... I think).
2nd I remember two scenes VERY VERY clearly. There is a family reunion or party or something going on, the grandfather offers to take the boy on a fishing trip, just the two of them, which the boy happily agrees to go on. The Sister overhearing this asks to go too, but the grandfather politely declines her, knowing the reason the boy wanted to go was to get away from his sister. She then asks please and proceeds to run her hand down his thigh and towards his groin. Sufficiently freaked out he takes her out of his lap and says no. Second scene I recall is the boy asking to get a Popsicle from the freezer in the basement and is told yes, the sister asks the other parent since by now one of them has been trying to keep the girl from being alone with him. However the other parent agrees and she runs off too quickly to hear the other parent objecting. Now wife/husband is confused why one can have a Popsicle but the other can't and he/she tries to explain the strange things they've noticed.
That's unfortunately all I remember about the show. I saw it long ago and it's certainly a late 90s or very very early 2000s, though it might be a rerun as well, so don't focus too much on the timeline there.