Can I use Hysteric Party's effect on Harpie Lady Sisters?

Unfortunately not. The reference to "Harpy Lady" in the text of Hysteric Party refers to Harpy Lady #1, #2, #3 etc. Harpie Lady Sisters is a seperate card altogether.

If it was summoned correctly first, yes you can. I think.

Well, unfortunately there are no rulings on the Party or any of the other cards referencing "Harpy Lady", I did see this card:

Harpies' Hunting Ground
Field Spell
When "Harpie Lady" or "Harpie Lady Sisters" is Normal Summoned or Special Summoned, destroy 1 Spell or Trap Card on the field. All face-up Winged Beast-Type monsters gain 200 ATK and DEF.

This looks to me as though Harpie Lady Sisters is treated as a separate card than Harpie Lady. Also note that #1, #2, and #3 all include the text that they are to be treated as "Harpie Lady", and with the structure deck focusing so much on them, I think we would have been given an update, similar to Archfiends as to what qualifies as a Harpie Lady.

That's my 10 bits 8^D

Hysteric Party doesn not special summon Harpie Lady Sisters.

It says to summon as many "Harpie Lady" cards from your grave as possible. This is where it gets tricky.

In the newer cards (POTD and up), when it says a "something card", that's short for "a card with something in its card name". If its wants something specific, it would say "a something from your deck".

Since this card came out shortly before POTD, this probably doesn't apply to Hysteric Party and it can only special summon "Harpie Lady" from your graveyard.

Luckily, that includes Harpie Lady, Harpie Lady 1, Harpie Lady 2, Harpie Lady 3, Cyber Harpie Lady and Harpie Queen.

No, I would say that the newer standard of "..." cards meaning cards with "..." in their name definitely applies here. If it only meant to work on the Harpie lady and her 4 aliases then it would just say to special summon as many Harpie Ladys as possible since they are all considered to have the exact same name.

Besides the "..." cards format has been around since archfiends came out. An "archfiend" card is a card with archfiend in it's card name (as well as about 12 other cards that should have had that word in their name but the theme didn't exist when those names were translated). It's just that later when more of these themes started popping up that they brought this to attention.