So, you can invite your mum and sister but his are excluded!!
Can you really not see why that is particularly unkind and hurtful? Of course you can, but you'll probably press on ahead with it anyway.
Nobody is saying you must have a big wedding, not at all. But leaving his mother and his immediate sibling out like this is very poor behaviour from you and likely to prove divisive in the future. Unless a humungous drip feed about how abusive they apparently are is shortly to emerge (which won't be believable).
You invite your mum and your sister. He can surely invite his own mum and sister too. Meet everyone else at the pub/restaurant afterwards if you are so inclined.
You don't say where any fathers feature in this, so I'd guess that they may not be on the scene.
Anyway, yes, you are being totally unreasonable here. I would be very upset to be excluded from my child's wedding in this way so, in the absence of any information to the contrary, I am team MIL/SIL here.
You are choosing the way to get yourself off on the wrong foot with your future in-laws. The damage from that may well not be reparable if you proceed with this.