With the release of "Harpie Queen", including this card in your Deck is strictly optional. However, the Trap-negation ability of "Triangle Ecstasy Spark" may prove very useful if you are unable to use "Harpies' Hunting Ground" to destroy your opponent's Trap Cards. If you happen to have many copies of "Elegant Egotist" and have already Summoned all "Harpie Lady" cards out of the Deck, "Harpie Lady Sisters" is the only other monster that "Elegant Egotist" can target.
You should not have more "Harpie Lady Sisters" than the number of "Elegant Egotist" and "Hysteric Sign" combined in your deck, since it is impossible to Summon without them.
This card is a dead draw as it cannot be Normal Summoned, and can only be Special Summoned by the effect of "Elegant Egotist" which is able to Summon this monster from the Deck. Cards such as "Magical Mallet" and "Card Trader" can return this card to the Deck and offer replacement cards to prevent the disadvantage of having "Harpie Lady Sisters" in your hand.
It can be discarded to trigger the effect of "Harpie Channeler", but doing this prevents it from being Summoned from the Graveyard since it was not Special Summoned properly with "Elegant Egotist" beforehand.
Use "Harpie Lady 1" with "Elegant Egotist", so when this card is Summoned, "Harpie Lady Sisters" will start off with a better attack strength.