You need to let this go. I understand it's brought back feelings of rejection, so I would suggest that your DH goes for counselling to help him manage these feelings.
But you've said it's not about the money. If that's true then why are you discussing challenging the will after he's died? Doing so won't cure the feelings of rejection, will it? The simple fact is that your DH has no right to the money and your FIL is free to leave his assets to whomever he chooses.
You sound very judgemental of your SIL and I am not sure why. She used to be an alcoholic? Good for her for quitting. She made some bad decisions in the past? Haven't we all! But she's stepping up now and being your FIL's carer. Do you have any idea how hard that is?
It's 24/7, 365 days a year. You don't get time off. You don't get to go 'home' from it. You get next to no support from social services because the money isn't there. They don't care if you are on your knees with exhaustion and stress - and forget trying to get respite care. In view of everything she is doing now I am surprised that you can dismiss it so casually and insist instead on clinging to past misdeeds. I can only assume the reason is because you don't really understand the relentlessness of being a full time carer to a relative.
You also sound judgemental of the fact that she gets paid the carer's allowance - but she is his carer, so why shouldn't she get it? It's £200 per week. Even on a standard 40 hour a week full time job that's only £5 per hour when minimum wage is £7.50! And trust me, she'll be doing more than 40 hours a week. Your FIL requires care. Care has to be paid for - either by selling your assets and getting the state to put you in a home. Or by employing private carers and making financial arrangements that mean they will look after you. Your FIL is simply doing the latter. If he's leaving her the other half of the house then it's still unlikely to cover the actual cost of what his care would have been in a home (for context you are looking at an average of £500 per week for a full time care home - and you would need to pay for 'extras' such as clothing, toiletries etc.).
It's a shame that your feelings of dislike for your SIL are such that it's blinding you to the fact that she's the one who has given up her life indefinitely to care for a bed-bound elderly man who requires round the clock attention. And that you feel that £200 weekly is ample reward for this. If that's the case then why don't you or your DH go and care for him and 'earn' all that money?
Finally visiting twice a year is nothing to be proud of. My parents are a 5 hour drive away and I visit every 8 weeks and stay for a long weekend. It's not difficult to do if you choose to make the time for it. It's fine to not want to visit, but don't try and dress up two annual visits as evidence of 'support' because it really isn't.
So with this in mind, take a long and hard look at your motives, because at face value from what you have written it sounds like the very definition of entitlement. You and your DH are suggesting that these twice yearly visits are somehow comparable to someone who is providing round the clock care, and that as a result assets should be split equally. I suspect that if the boot was on the other foot you might feel differently.