Yep--it's a tough situation and I certainly feel your pain.
But, remember, she IS his mother.
Try to NEVER say anything you wouldn't want him to say about you own mother. Tough, I know.
Look, she is 81. She may not be around much longer.
Maybe you all need to honestly discuss what the best option would be.
What IS your bottom line? Do you want her gone no matter what? Do you need more room? Do you want her with another of her kids? Try to determine what the real problems is here: personal conflict? Space? Financial difficulties? Decide what your bottom line is and discuss it with your husband. If you want her OUT and he refuses, then you've gotta decide how to cope at that point. Is it a deal breaker? Would you leave? Can you manage. Sounds like he's set on her staying, so you may just have to get used to it.
Scenario #1: Look into an assisted living facility and let her money pay her way. Be warned $300,000 does not go too far when the cost may be 3-7K /month -- maybe 4-6 years. Honestly, from the post, it sounds like you want her out--not moved with you to a larger house. Maybe this is the route to go. MAYBE at that point, another sibling will step up and mover her in with them?
Scenario #2: As her to start contributing financially what she can per month, from her SS, savings, or pension or whatever she has coming in.
Scenario #3. If you can afford a larger house, start looking! The housing market IS turning and it still may pay off for you since you may be able to get more space for a better price.
Scenario #4. carry on with your plans to have another child IF you're sure that's what you want and it's not a topic being used to prove a point about her needing to get out. Lots of people have raised 2 kids in a 2 BR house and have lived to tell about it. You'll get that bedroom back eventually for your use.
Scenario #5. Maintain the status quo. Maintain respect, love and care for this woman, regardless of the lack of interest of the other siblings to pitch in and help out. You will most likely bite your tongue so many times that you feel it's hanging by a thread. Keep biting. Maybe you're going to have to accept this situation and deal with the fact that she's not going anywhere. Do you really want to be the daughter-in-law that "kicked mom out" or made her son "kick her out"?
I would advise your husband to very carefully keep track of ANYTHING he does with her money, so the siblings have NOTHING to say when, God forbid, something happens to her.
I would advise you all to discuss who is the executor of her will, how money will be distributed, etc., NOW rather than later. Family should not be about $A=$B, etc, but it often boils down to that when there is a death and money is involved. Best of luck to your family.