bigger bike? My wife enjoys riding but hates the comfort on our sportster. I have installed a sissybar and a mustang touring seat but she still gets uncomfortable on a long ride. We our absolutely too broke and can't afford a bigger bike at this time.
I have a better idea
an 07 HD 883C's are selling used for around $12K
sell it for $12K and for that much money you can buy a better bike that's passenger friendly such as a Yamaha V Star 1300 Tourer at $7599 brand new, or a Honda VTX1300T at $8995 brand new
Get a proper 2-up seat. The mustang is good for the driver but is seriously lacking for passenger comfort. Rule of thumb, the uglier it looks the comfier it'll be.
.. try j&p cycle. online or print catalog. all kinds of seats. you probably already know about these guys if you have a mustang seat, but hey its worth a try. keep momma happy, she's the gatekeeper.