Since I haven't owned a television for about 22-23 years my suggestion may seem weird but it's an honest one. Assuming it is physically possible, would your mom be willing to have the television taken out of the house or at the very least hidden in a closed closet today? It could go back on Friday if she is willing, but if it is gone then there cannot be any problem. I would add, however, that your SIL should be informed that there is no television in the house before tomorrow so she can make a decision about coming. I do not, and would not, suggest this as a passive-aggressive move, but simply as a good idea. If Mom wants to host the family in a certain way, that is, as a family gathering without competition from a television, but she knows that SIL is a football fanatic, then Mom does have the option to rearrange her home in a way that best suits her choice.