Can my siblings sue me for my mom's home?

I don't know the value of the home, but were Mom to have in home caregivers for 5 years and pay for them that would be a whole lot of money.
Your Mom signed this home over to you more than five years ago and this likely takes care of medicaid lookback if that is something Mom will need, but I would go to an Elder Care Attorney to make certain of that.
This home is yours and has been for these five years. Your siblings can sue the Mayor of New York if they want; anyone can sue anyone else if they want to pay upfront fees and your own fees if they lose. You should pass their intentions past the elder law attorney you see. Take your deed along with you.
Wishing you good luck. Whether they swoop in now, or would have waited until Mom passed they would have been swarming you at some point anyway. See your attorney; this is work that is part of Mom's estate work and care if you are the POA, so this can be paid for with your POA through Mom's funds. You will need advise now to get her settled.
I wish you the best. I cannot know if once your siblings DID know the home went to you and BELIEVED that it was for life care of your Mom. That is all well and good, but things change. Five years of care is a long time. We all have limitations. See the lawyer to get advise. Hope you'll update us.

Spend some money on an elder law attorney just to make sure everything has been done legally. If mom signed only a quit claim deed there may be some problems. No surprises is always best. Do not tell your sisters.

Was mom considered competent when she signed whatever on the house to you? Or could sissies claim you coerced her?

Have you checked the Medicaid look back laws in your state? Usually it is 5 years, but check to make sure. It would be best if the property transfer was at least 5 years (not 4.5). Do you have an attorney or did you have one to help you? It may not be necessary but it may be useful now with the threats from your siblings, just for your peace of mind. They shouldn't have a case, but you never know what a skilled and ruthless attorney may be able to accomplish. If they do hire an attorney, I would advise you to do the same.

MargaretMcKen is correct, a bigger concern is whether your mother will need Medicaid now (or soon) and what the Medicaid financial application "look back" period is for your state. You need to find this out by either going to the social services website for her county or talking to a Medicaid Planner or elder law/estate planning attorney. You will need to make sure that you've not been inadvertently managing her other financial affairs in a way that may delay her acceptance (like co-mingling funds, getting paid in large sums of money that look like gifting, gifting of other properties and assets, etc.) Again, this forum isn't the place to get the professional and exacting legal guidance you need.

Two things:

Your mother ‘signed it over’ can mean a lot of different things to people who don’t understand the local law on both property and Medicaid. Is the title now officially registered in your name, and your name only? Or did mother just sign some sort of paper saying that you could have it?

Secondly, is the date of all this past the local Medicaid look-back period? It could affect the money substantially.

Let your sisters get a lawyer then. Anyone can try to sue someone. It doesn't mean they will win. Your mother signed this property over to you free and clear five years ago. It is not part of a will or inheritance because she's still living. It's your property and not a damn thing your sisters can do about it. You are the one who's been caring for mother. Not them. Remember that.