Hi! I'm J..
That's terrible! You are completely right to want to breastfeed Joshua to 18 months. I'm a breastfeeding mom and I feel for you.
I suspect that your husband, mother and mother-in-law are jealous of the relationship you have with Joshua. (I've experience that.) There is something very special about it. It's a private club that they'll never be a part of. It's up to them to engage the baby in a way that satisfies him. It's obvious they aren't being creative and can't think of anything to do with him and are blaming you which is very unfair! (I've experience this, too!)You can't change their behavior or comments so all you can really do is ask them to not make comments in front of the baby. Babies absorb positive and negative feelings from the people around them.
I don't know if this will work but maybe a book of activities for babies and toddlers would help. Leave it out in the room with the 'trio of unbelievers' when they are in charge of Joshua and tell them to try something in the book. Gymboree has a great book called 365 Activities You and Your Baby Will Love. Maybe the librarian in your area can make some more suggestions.
I'm sure you've presented the evidence to them that breastfeeding is best. Most Americans have a hard time with the idea of breastfeeding past a year. It's unfortunate that breasts are sexualized in our society instead of being seen as a source of nourishment and comfort for babies and toddlers. Sorry to write so much, but this is an important issue to me! You are doing the RIGHT THING!!! I'm rooting for you.