How do I get help for my dad?

You begin by staying as far away as possible from your dad, along with your children as well. Something is very wrong with your dad now, if he's made 2 sexual advances towards you. That is disturbing to say the least. He needs a psych evaluation for starters, but you're right, he probably won't admit to any wrongdoing, so you now must protect yourself and your precious twins from any harm that your dad might inflict on you all.
Have your sister help you find a place for you and your children to live, and move out ASAP. Your dad will be fine on his own now, and hopefully he will get the help he desperately needs. But if he doesn't, make sure you and your children stay away from him. I am speaking to you as someone who was sexually abused by my father for many, many years, and my abuse started at age 5, so please protect not only yourself, but also your precious children. I'm sorry that you are having to deal with this, and for the loss of your mom. May God bless you and your family.

At 73 he should be used to having the amputation and be able to do for himself. Unless there are other health problems? Is he a diabetic? Who manages his medications? If he manages his meds, is he taking them correctly?

I agree, stay away. And tell him why.

Is your Dad drinking? Or drinking more?

As a teen I used to stay out of grabbing reach of certain friend's Dads. Especially if I saw a beer in their hand.

Can he stay by himself? If so, check on him by phone for now.

Or is he dependant on you for daily care? If so this will need to be escalated - as would any other circumstances of a vulnerable elder left alone if the main carer has left (due to emergency/other/safety issues).

It may be some sort of severe grief reaction, causing confusion. This would need medical assessment.

So sorry for the loss of your Mother.

Stay away from your dad! Do NOT allow your children to be near him.

I am so sorry for the loss of your mom.

Call his doctor. Get them to help but you don’t have to be near him.

Stay away and keep your children away from your father. How is he in possession of his mind and making sexual advances towards his daughter?

Call his doctor on the phone and alert him to your father's new and inappropriate behavior. Then find somewhere else to live permanently that's safe for you and your young children.

Grief does not manifest itself in such behavior, but dementia may. He needs a cognitive evaluation at the very least, and a workup by a geriatric psychiatrist.

Please don't place yourself or your children in harm's way again in a heroic attempt to further care for your father. Do it from afar now.

My condolences on the loss of your mother.

Good luck