Struggling with questions of obligation and possible feelings of guilt for not helping my dad. So much more to this dilemma.

I will try to make this as short as possible. My dad has early stages of dementia with a long list of health issues. The most recent is kidney disease due to a new diagnosis of autoimmune disease. He has his wife who's 76 he is almost 80, they are on Medicare and between pension and ss they bring in about $5000 a month. They only have a mortgage maybe $1000 a month no car payment so just their basic living expenses as far as I know. I have 2 sisters and his wife has 4 kids. 1 of my sisters is estranged due to them telling her years ago she is going to hell for being gay. I have been back and forth due to my issues with them as a child. I was molested by my mom's boyfriend and physically and emotionally abused by my mom and my dad and stepmom just looked the other way. My dad started to have mobility issues in 2020 and I began to check in with them more as I was feeling guilty for not always being in their life. I lived away for 20 years and moved back. Stepmom started to guilt me in to helping more such as buying his depends since he is incontinent and asking for me and my sisters to pay for someone to watch him while she does recreation stuff for herself to get out of the house. I instead got his VA benefits he was never in a war but was in the reserves. They have provided someone in the home a few hours a day a few times a week. Well that wasn't enough for my stepmom she started to ask for more. My husband and I have been married 5 years we are both divorced so we basically started over financially when we got divorced. Our divorces were not in our favor and we came into this marriage with nothing so we are trying to save as much as we can for our future. My husband is 56 and I am 48 so we are trying to make up for lost time in saving. Plus we want to buy long term care insurance so we can make sure we are OK in our golden years. Neither of us have a job with a pension. Plus we don't have children. So I really don't feel comfortable helping them out financially. Plus I work a lot and don't feel up-to going over and babysitting. 5 years ago I also found out that my mom lied to me and my dad and he isn't even my biological dad. Though that doesn't really matter since he is the only dad I have known. My issue is they had many years to plan and prepare for their future and I'm not sure why they have never saved any money. I have no one to fall back on and my sister who is stepping in to help can't afford it herself. I know she has zero saved for her future and that worries me, she is 9 years older than me. Another issue that i just recently started therapy for. My stepmoms grandkids have come out and said my dad molested them when they were younger so her kids and grandkids have washed their hands of my dad who they thought of as their dad and grandpa. It's all such a bizarre sick situation and I don't like to even think of it. I am angry over it and angry at him but still feel some compassion since he is declining. I am just so unsure of what I am expected to do at this point and feel so helpless even to myself. Any advice or even emotional support I will gladly take. I now understand why my stepmoms kids are no where around their house or even offering their mom some respite. He is back in the hospital again with the kidney issue and recently started immunotherapy infusions but I don't think he will be around for much longer. The Dr said any illness he gets he will succumb to since his immune system will be wiped out. He will be discharged soon and I don't see how my stepmom can take care of him fulltime, he can't walk now.