All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
One of your rights as a wife is to have a separate residence instead of living with your in-laws. Please refer to Fatwa 84608. You can discuss with your husband about moving into a bigger house where you would have a separate section for yourself, or about him living in one house and his mother and sisters in a another house nearby so that he would be able to take care of them accordingly.
It is the duty of the husband to provide for his wife and children according to what meets their needs. The obligation to sustain them is given precedence to providing for his mother. It should be noted, however, that the son is obliged to provide for his mother if she needs financial support. Please refer to Fatwa 270145.
Also, if the husband fulfills the financial rights of his wife and children, then he has the right to support whichever of his relatives he wishes. If the sister has no one to support her financially and no wealth by which she can provide for herself, then her brother is obliged to provide for her according to some scholars. For more benefit, please refer to Fatwa 263056.
On the other hand, there is no harm in advising your husband to save money. The Prophet, sallallaahu ‘alayhi wa sallam, said to Sa‘d : "It is better to leave your heirs free of need than to leave them poor, begging from people."
If he refuses to do so, you are advised not to allow this to come between you and your husband. Perhaps because of his generosity and support of his family, Allaah will generously reward and compensate him. Allaah The Exalted says (what means): {But whatever thing you spend [in His cause] – He will compensate it; and He is the best of providers."} [Quran 34:39]
Abu Hurayrah narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: "Charity never decreases wealth." [Muslim] Allaah may also reward him for his dutifulness towards his family by protecting his children when he has passed on. Allaah says (what means): {And let those [executors and guardians] fear [injustice] as if they [themselves] had left weak offspring behind and feared for them. So let them fear Allaah and speak words of appropriate justice.} [Quran 4:9]
This was evident in the story of the two young boys mentioned in Soorah Al-Kahf (chapter 18). Allaah The Exalted says (what means): {And as for the wall, it belonged to two orphan boys in the city, and there was beneath it a treasure for them, and their father had been righteous. So your Lord intended that they reach maturity and extract their treasure, as a mercy from your Lord. And I did it not of my own accord. That is the interpretation of that about which you could not have patience."} [Quran 18:82]
Allaah Knows best.